GBRs are better but you have to have pristine water for them. i went through 6 before i got a pair.
you could do kribs, but get maybe just 2 females. they get mean when they breed...same with the GBRs.
and i think you need to seperate the fish into levels, so you have movement everywhere
GBRs will tend to stay near the bottom
swordtails...everywhere *rolls eyes*
tetras normally near mid-top