What I tend to do, is go with fish from one area of the world.
Also, I like to have a few feature fish, some dither fish, and usually catfish.
I find more of a lesser variety is nicer as it looks more natural and doesn't look as full and confusing.
If you want, what I like to do is put in a few fish you wont see to often. That way there is always something to look out for in your tank! :lol:
My main point though, is less variety, more of the same.