Hello and welcome to MFK.
Couple things I noticed... water conditions for african and american cichlids don't matter much unless your catching the in lake malawi, lol. Most of these fish are kept at LFS's in the exact same water conditions... my african and americans are kept in the same water too. The biggest issue I would say with mixing the two is the africans will tear your other fishes fins up pretty good... why would anyone want that?? I keep both, just in seperate tanks. Feeding is another issue... africans, Mbuna in perticular, need to eat veggies as pretty much 95% of their diet. Americans benefit from a good source of veggies, however eating a lot of protein is also very good. I feed my africans veggies with a treat of blood worms once every other couple weeks. If they were in the same tank as my american cichlids they would be eating a ton of proteins, which could lead to malawi bloat. Again, the water has nothing to do with why they are in seperate tanks. The puffer, from what I know, should be kept in brackish water only?? Welcome again and good luck.