The serpaes are being nippy for 2 reasons... first, they are a really nippy species of tetra, and second, they aren't in a big enough school. In a bigger school, they'd have more of each other to nip, and wouldn't go after your other fish as much, but with just the 2 they nip a lot more. If you don't want them, can you take them back to the store? Otherwise, you'd have to either get more of them, or put just them and the snail in the 10g so there isn't anyone else for them to nip.
The only fish you have in the 70g that could go into a 10g are the serpaes, the crab, and maybe the swordtail but he will really get too big for it. And 2 serpaes in a 10g with those fish will do a lot of nipping.