1.) The girls are added to your collection permanently. If you don't renew your subsription you go back to one demo per day which stays with you for five days.
2.) It is random.
Just like you I'm not interested in all the shows (I'd say around 55% interest me at the moment) and I went ahead and got a subscription for a month. I got some good, some howlers and some that were just average and that was the problem for me. Now I just use credits. Might be worth giving it a month and see how that works for you (plus you get a couple of freebies if you do!).
If it really is only 30% of the shows that interest you, then the economics are pretty marginal. But you do get the shows to keep so if you don't like them you can always delete them and maybe redownload them in the future if your tastes change. Plus you can also use credits to buy any you really want that come up on special.