As everyone said, 55-75g isn't big enough to keep either of the bass species that you are interested in long term. Even with enough filtration to keep water quality high (say a large sump or something) both a smallmouth or largemouth will far outgrow such a small tank. 2-3 feet.
You could keep one or more rock bass or shadow bass in a 75g.
You have several choices of sunfish that will work in either a 55g or 75g. Green sunfish, longear sunfish, pumpkin seeds, just to name a few of the larger species. Smaller species would be banded sunfish, black banded sunfish, and dollar sunfish. Probably a few others that I'm not remembering.
Bluegill are an option but can potentially get over a foot long and take over the tank, so you're choice, but I wouldn't personally suggest them. However if you're looking for something large and aggressive akin to a LM or SM bass, that would be a good choice. A smaller and slightly more peaceful choice with a large appetite and big mouth, very basslike would be the green sunfish.
What kind of water to use? Same as any other fish, dechlorinate it and put it in.
Chemicals to change water? Not suggested at all. How does your water come out of the tap? Native fish are extremely hardy and will adjust to just about any water. If the water needs buffering there are natural alternatives, don't use any chemicals.
What plants and food? Sunfish (this includes bass species) will eat all types of meaty foods. I feed mine carnivore pellets, supersoft pellets, worms, krill, live fish, live crawfish, live ghost shrimp, etc. Just try to vary their diet, but they'll accept all kinds of foods normally.
Sunfish species aren't sensitive or complicated at all, they're some of the hardiest and easiest to keep fish you can have.