Slow moving puffers shouldn't be kept with Mbuna, they get the tar beat out of them. They just aren't aggressive enough. If you do set up a puffer tank, don't plan on housing snails with them, or they will gorge themselves. Just drop a few snails in and they will gobble them up. The shells are really good to help keep their teeth trimmed.
No idea on the crabs, I've never kept them. I don't think they're suppose to be kept with fish, tho, then tend to try and catch them to eat them.
Might want to check on the different water paramaters for each species, too. Puffers are a brackish water fish. They can survive in fresh, but not long term.