One thing is forsure, there is and always wiil be a chance that one or both of you could get hurt. You just have to accept that and not let it get in your way. It's better to have hit it and quit it, then to never have hit it at all (that is how that saying goes.... right?).
If she's rubbing her ass on you, there is interest on her part. Next time she throws that at you pinch it and say, "that was just a warning! Next time you do that I'm going to grab it with both hands"! And then wink at her. She sounds playful, so be playful right back. And always remember to display confidence, chicks dig that the most.
Don't go overboard into cocky, nobody likes that. But if you give off the impression that you're scared, or don't feel worthy, she'll lose interest. And remember, if she teases a lot, it probably indicates she's interested, but also scared of getting hurt.
As for the shy girl, well we certainly don't have the insight or authority on who you should pick or which would be the best choice for you. That is something only you can decide. Have fun!