Lots of good advice already said so I won't waste time repeating it
I think I'd figure out first what he might like to put in. If he chooses the aggressives he's going to be a little more limited in what types he can put in and generally he'll have less fish because even the ones that don't grow huge, like the oscar, are still going to grow larger than your average community fish. Some of the aggressives seem to have more personality, bond with their owners, and have habits that make them interesting to watch though.
On the other side there's the community fish. In general most of those you find in shops grow about 2-4" or so so he could put a lot more of them in and he'd have a larger selection to choose from.
Other than all of that you actually take care of a freshwater tank the same way you do salt! Set it up, set the temp, cycle the tank, only get a few at a time (go with 1-3 for 10 gallons I'd say unless they're tiny like neon tetras or bumble bee gobies, then you can maybe add a FEW more than that but don't push it), and do your partial water changes.
The livebearers, guppies, mollys, platys, and swortails most commonly, can be pretty interesting to breed but keep in mind that they're very prolific!!! It's not hard to have your tank overstocked in no time with these guys!!