Some people wouldn't recommend Platys or Mollys in a 12g tank. I think it is Ok to have a couple, but I wouldn't put them in with a betta. Mollys and Platys are live-bearers, and those tend to be nippy. My betta used to live with a Swordtail before he had to move because his fins were getting nipped.
I also don't generally recommend RCS with bettas, because some bettas (there is no way to know until you try) will eat or kill the RCS. RCS are pretty small, and they have bright colors. Even the bigger, harder to see shrimp like ghost shrimp are in danger when it comes to bettas (Tony tried to kill one of my GS when I put it in for the first time).
Snails are generally safe when it comes to bettas, especially snails with "trapdoors". Each snail requires a little bit of research, though. It is a good idea to know what kind you are going to get.
FireOasis - 3g
Zachary the Halfmoon Plakat
CichlidHaven - 55g (under construction!)
0.0.7 Black Widow Tetra
1.0.1 Convict Cichlid
0.1.0 Firemouth Cichlid
HermitBase - 10g
Zephus & Monoculus the Purple Pinchers (Coenobita clypeatus)
x1 Convict Cichlid Pair (actively breeding)