I agree with the above, nothing that ends up over 8 ". Preferably smaller)
As far as Central Americans go, I have had long term success keeping pairs Amatitlania species in 75s.
Amatitlania mynae.
Panamius panamense.
Amatitlania nanoluteus
With South Americans there are many more options
Most Gymnogeophagus have worked well
As have some Cichlasoma
I picked up some Herichthys minckleyi from imperial tropicals and have them in a 75. Loving them so far, although they are fairly aggressive with each other. Once the Male pairs off with one of the females I will separate the others and leave the 75 to the pair. Not much info out there on them but they should stay under 8". I'm expecting 6.5" to 7.5" out of them.
I'd go for a pair of salvini. Nice and colourful especially when breeding and you can have plants with them which will make the tank look fuller. They are so territorial when they breed you won't be tempted to add any other fish either.