as far as i know you can because it says that you may play them when you would be sent to 0 or less or durring your turn... say someone hits you for 3 you drop 2 first aids the next turn putting you at 16.. it at least doesnt say anywhere that you cant go above 15 counters
Beer Money cards have their own back, so you can tell when someone has a hand of Beer Money cards if you mix the decks. (It makes it easier to separate them back out, though.)
Yes, you can go over 15 counters with First Aid. That's pretty much an open invitation for others to smack you right back down again, though...