Amanos would be about 2" as adults, a bit big for dwarf puffers. Also, the breeding is rather difficult because the adults live in freshwater, but the shrimplets need fluctuating amounts of salt to survive. I read a good article from a guy that successfully bred amanos by transferring the female to brackish just before she let the shrimplets loose and then put her back in freshwater. Then he would adjust the salinity pretty much daily. I can't remember where I found it, but I think I just googled breeding amano shrimp.
Nerites can also breed in brackish. In fact, many need brackish to breed. However, I believe they might be tricky also, but I haven't looked into it.
If you want to breed shrimp and snails for the dwarfs to eat, I would stick with cherry shrimp and pond snails or ramshorn snails (all freshwater) since they are easier to breed and don't get too big for the dwarfs to handle.