Aesthetics aside, there are a few boxes to check for fish that are compatible together and it's not necessarily their native region.
I'm purist leaning also, more so in some tanks than others. In other words I have display tanks where I'm more restrictive, but I've also had all purpose tanks more for convenience. And like most anyone, there was a time I put all kinds of stuff together. So I've tried a lot of combinations and seen a lot more, some of them weird ones I wouldn't do myself, but they can work.
There's a pretty good overlap between new worlds and old worlds you can keep together. There are sensitive species and species at opposite ends of pH and hardness you wouldn't do, but the range of comfortable water conditions overlap in many species. Within this group temperature is another consideration. Seldom is feeding preference the issue some make it out to be, not for majority of freshwater fish if you're primarily feeding pellet or flake in the first place. In many cases, then, it comes down to temperament and behavior compatibility. Not just aggression, but calm fish vs. busy or boisterous fish. Some slower moving species aren't especially happy with a bunch of hyper activity in their tank.