That other thread that was mentioned can't be beat for general advice and the others around here have added a lot of good advice specific to your situation but, as usual, I have my own take on it.
First of all, you can ask your hostel about their "guest policy" if you want, as brother JB suggested, but I seriously doubt they'll look too favorably upon it if they even know right away what you're talking about. Hostel-type places just aren't the sort of places where guests normally bring back working girls. Beyond that, I'd imagine that for that $13 you're probably not getting a private room so, unless you're into putting on a show for your bunkmates, you probably wouldn't want to bring a chica back there with you even if it were allowed.
That leaves you with having to get a second room, either a ST one or a second room for all the day if it is cheap enough, IF you're going to be doing anything other than MP's. Now we're talking $13 for the Pangea and another $15 for the ST or cheapie tico hotel or $28/nt and at that point you're really not talking about that much less than a room at the Castillo or perhaps the Morazon. And for that little bit of extra money you'll get to sleep in your own room without having to worry about snoring bunkmates (or theiving ones), you'll have a much more pleasant environment to do the deed with your chica and as an added bonus you'll find like-minded gentlemen around for comradery and guidance in your P4P endeavors. Of course, you can always get that last part by just leaving your hostel and walking over to the the Hotel Little Havana, Sportsmans or Blue Marlin, but these more "monger-oriented" hotels will have it right on the premises as well as being much closer to those other spots. Personally, if I were you, I'd pay the little extra for these "monger-hotels" while you're in SJ doing the monger thing, and save the hostel-experience for when you're elsewhere in the country doing the eco-tourist/backpacker thing. Or maybe, if you want to save a little money, do the hostel thing for ONE of your 2 nights in SJ and just do the MP's that day and then spring for a room at one of the aforementioned "monger hotels" for the other night when you do your bar pick-up thing.
A word about Cali's fear of Cryptosporidiosis, IMHO that might be a little overstated, but I wouldn't be too impressed with Pangeas listing of a pool feature as one of its amenities for other reasons. It can get damn cold in SJ at this time of year, at least at night. Even if it gets warm during the day, that pool is probably gonna feel like ice-water. There is a reason most hotels in SJ don't have pools (or, for that matter, a/c). Despite the extreme southern latitude, the city's altitude keeps the area a lot cooler than most northern visitors would expect. I expect that few if any locals would go in pools at this time of year and not even that many northern visitors either. OTOH, the Pangea pool area may not be such a bad place for scoping out young gringas sunning themselves in their bikinis
Regarding Steven1's advice on transportation, using the shuttlebus company he listed is much more advisable than paying the bigger bucks for a taxi or the Pangea's shuttle service and if you want a no fuss way of getting into town that is certainly the way to go. OTOH, if you're a little more adventurous and want to save even more money, that might be the time to try out CR's public bus system. For ~75 cents and not that much more time or difficulty, you can catch a bus from in front of the airport (leaving every 10 minutes) into town (transit time 30 minutes) and then for just one more buck grab a cab from the bus stop downtown to your hotel. For more info, see my posts in this thread:
http://www.costaricaticas.com/phpBB2/viewtopic.php?p=218330. For some people going this route may seem cheap just to save $8. Personally, I do it as much for the grass-roots experience as anything else.
Regarding the Grayline Bus Pass mentioned by Steven1, even for one who expects to do a lot of travel while in CR that is not always the deal it seems. First of all, even if you plan on traveling between locations every day you are there, IMHO, you'd be much better advised to reduce your itinerary, relax and enjoy a few less places better rather than spend 3-4 hours per day practically every day driving around in a shuttlebus. That means spending at least 2 nights on average at each destination if not more. So for a week long visit that would probably mean you'd only be to comfortably fit in 2 locations other than SJ or 3 travel legs (SJ>Dest#1, Dest#1>Dest#2 and Dest#2>SJ). This is even more so the case for you since you already said that the first 2 of your 7 days will be spent in SJ, leaving just 5 days to visit the rest of CR and use your Bus Pass. At the usual $30-40 per individual travel leg, at best you'd probably only be breaking even on the Bus Pass.
Lastly, regarding your itinerary, there is no way that you'll be able to spend 2 days in SJ and also see all the places you mentioned in one week. Don't feel too bad about that though because, after you go to CR one time, you'll probably want to come back any way. Of the places you mentioned the easiest ones to get to would be Arenal/Fortuna and Manuel Antonio/Quepos.
I suppose it can be done for just one night but traveling to Monteverde requires a bus ride up a rough dirt road and the arduousness of the journey itself IMHO makes it not really worth it unless you're planning to spend at least 2 nights there. If you do decide to make this one of your destinations, I highly recommend you use the taxi-boat-taxi or the horse-boat-taxi tour to take you to Arenal next. For more info on this area as well as those 2 onward transportation options go to
http://www.monteverdeinfo.com/
Similarly, Mal Pais is a great beach destination but requires a combination of bus, ferry and bus to get there, making it a longer harder trip than to say Jaco or Manuel Antonio. Grayline doesn't even cover the Montezuma/Mal Pais area so you wouldn't be able to use their Bus Pass for that. Shuttlebus/Interbus has a shuttle to Montezuma leaving every day at 8AM for $37-39 or you can take the public bus/ferry combination for $10 but either way you're looking at a 5-6 hour trip to get there, thereby taking up most of one of your limited days. Worse yet, there are no direct shuttles from Montezuma to anywhere else other than back to SJ, so you'd probably have to take some combination of buses and shuttles to travel anywhere else. There are direct shuttles from MA to Montezuma, but are you going to limit your 2 non-SJ destinations just to the beach. And there is a direct shuttle from Monteverde to Montezuma. Frankly, as nice as this area is, I'd save it for another trip when you weren't trying to see so much. For more info on the Mal Pais/Montezuma area, I recommend you go to
http://www.nicoyapeninsula.com
That brings us back to the 2 "easier" destinations I mentioned earlier. Grayline has 2 shuttles/day to MA for $35 or you can take the public bus which leaves SJ every couple of hours for <$7. From there to anywhere but back to SJ by public bus becomes problematic but the daily Grayline shuttle to Arenal is another $35 ($45 for the shuttle from MA to Monteverde). We're now up to $70-80 in shuttle fares. Even if you take the shuttle back to SJ from Arenal for $35, you'd still be $17-27 ahead of the cost for their weeklong pass. However, I think a much more interesting way to go (and a great way to wind up your trip) is to use
https://www.exploradoresoutdoors.com/ to get back to SJ. Their Pacuare rafting trip costs $100 but you'd probably want to take that or some similar rafting trip at some point during your visit anyway so it would almost be like you're getting free transportation back to SJ. Another possible way to do this in the opposite direction is to take either the rafting trip or the Grayline Arenal tour ($99 including meals, tour stops along the way and hot springs admittance) to get to Arenal (staying there rather than returning to SJ if you go with the Grayline tour), then shuttle to MA and shuttle or bus back from MA to SJ or the airport.