Thirded -- ditch the Acarichthys heckelii. Even a single specimen will grow large and boisterous. They are not a good fit for a 4' tank.
If you don't want to do the orange head geos and would rather have some Central Americans, I'd also recommend rainbow cichlids (Archocentrus multispinosus). They are a small growing, peaceful fish that have nice mustard yellow coloration and will turn bright yellow/black when spawning. I have had pairs form and spawn in a 55 gallon without giving other fish too much trouble. You can also mix them with smaller, more peaceful community type fish.
I would not add tinfoil barbs or silver dollars. They tend to grow too large for a 55 gallon tank. You can get some deep-bodied tetras or some type of cool pencilfish.
When it comes to fish who are forgiving of water quality, rainbows are probably slightly hardier than geos. Geos tend to develop HITH unless you keep up on your water changes and filter maintenance. This isn't to say that rainbows can/should be neglected or improperly cared for, but they tend to be tougher in the event that your water quality isn't up to par.