Welcome...
Kelbel, to Aquaria Central, home of the most loving fishkeepers on the internet
I said "loving" because when you ask about "hearty" fish, did you mean lovey-dovey cuddly fish, or did you mean "hardy" those that will survive anything?
All kidding aside, the choice of your first fish is an important one. One of my favorite, almost unkillable fish, is the checker barb, which would do very well for you if you had 6 or 8 of them. They have a beautiful subtle coloring, are hardy and easy to feed, do well in a range of water types, and are easy to sex (the males have a reddish dorsal fin with a black leading edge). To give your tank the showpiece, you could have a single male betta, which is quite stunning obvisously. He'll hang out around the top of the tank. then at the bottom, you could try a clown pleco, which is quite pretty, if a bit dull. I have two in a 55g tank and they haven't grown more than 2.5 inches, so it shouldn't get too big. Some of your cory catfish would be cute, but they do better in small groups of 4 - 6 and your tank will be too small for than.
This tank, for example, (and there are lots more like it) would accomplish a number things. You'd have fish who live at all the levels of the tank: betta at the top, checker barbs in the middle, clown pleco at the bottom. You get activity in the shoal of barbs, with a stunning specimen fish in the betta. You also get to play hide and seek with the clown pleco (plecos are largely nocturnal). The fish will adapt to a wide range of pH, the most important aspect of water quality being that you keep it stable. Temp requirements are minimal, you could set your heater so that you get 76 to 78 degrees and forget about it.
HTH
Val