Well, Fella's right and Wolf's right.
Most puffers (by about 2/3rds versus 1/3rd) are marine water fish rather than brackish/freshwater fish. So Wolf is right on that count. There are also lots of marine puffers in the trade, particular Arothron and Canthigaster spp. but increasingly various "Fugu" type puffers as well.
But Fella's correct that freshwater pufferfish are numerically the most common puffers in the trade. Since marine puffers aren't reef-friendly, they aren't all that popular. But the various small freshwater puffers seem to be steadily becoming more popular.
Back on topic: almost certainly mixing mbuna and puffers is a bad idea. Some people have done it with Tetraodon mbu. You used to quite often see this combo in Maidenhead Aquatics showrooms for some reason. Perhaps still do. Some Tetraodon mbu do seem to be peaceful enough to mix with mbuna, and in the wild they are found in at least Lake Tanganyika if not Lake Malawi, so their tolerance of hard/alkaline water is significant. Is it a good idea though? Probably not, because some Tetraodon mbu do become territorial and aggressive. Fish shop owners can perhaps pick out the peaceful specimens and remove them at the first sign of nippiness; home aquarists likely don't have that option. At the very least, T. mbu should be treated as a "tankbuster" fish that needs a giant aquarium, so on that tack at least it isn't perhaps worth keeping at home at all.
If you wanted to mix puffers and cichlids, the combos I'd suggest would be either chromides and figure-8s (in brackish) or kribs/rams and SAPs (in freshwater). Both these combos have been used with better than random degrees of success.
Cheers, Neale