What about danios?
We had a fish tank when I was a kid, and we had quite a few zebra danios. They're not difficult upkeep fish and they seemed perfectly happy. They also get along well in groups, at least, the ones we had did, and got along with the other fish. Tetra fish, likewise. Or Tetras? We had rainbow tetras ( I think, at least), and they got along well in the tank.
Are Betta fish these -- http://en.wikipedia.org/wiki/Siamese_fighting_fish
? Cos we had one of 'em. We named him Sinbad ;3. I suspect we weren't the perfect owners but he seemed happy enough, got along with the other fish and lived to a ripe old fishy age. I think that as long as you keep to the basic rules and don't do anything rash, they're not -that- much higher upkeep than other aquarium fish. I dunno though, I was just a kid at the time.
Certainly don't get sodding Angel Fish, though. My friend found that out the hard way. After being told by the fish pet-store people that they'd be fine in a tank with smaller fish (tetras and the like), they put them in together --- and carnage! The bloody things ate most of the other fish, and then one ate the other one. So, erm... maybe not angel fish.
~~~a.k.a The Dying Daylight in disguise, or Wino Horse... either way.