My betta has been housed with both a snail and shrimp (the ghost shrimp died because they escaped out the top) with no problems. He's very docile for a betta, doesn't even flair when he's cupped and set next to our other male's tank. He's never chased nor hunted any of his tankmates, in fact the ghosties were the aggressive ones when I had them.
I'm not worrying about cover, I'm a firm believer in making sure there is plenty of cover for everyone and the tank is naturally planted. I'm going with either flourite or eco-complete for substrate. The old tank is a 5 gal with loads of java moss, broad leaf anubias, anubias nana, riccia, duckweed, java fern reg, lace java fern and amazon sword. I'll be transplanting a good amount of it to the 12. Also, I'm going to add dwarf baby tears and christmas moss. That, and I'll probably be getting the faux driftwood below and attaching some of the plants to it.
Alright so platies! How many of them? Or would guppies be a better choice?