Just need some advice - Stocking & Filtration in a 55 gallon

[That weird guy]

I have currently have a lightly planted 55 gallon tank(plan to get more plants later on) with 2 marineland penguin 550 power heads a stellar air pump on 2 carbon filters and to my shame an over the back meant for a 5-15 gallon tank(all the appropriate sized over the back filters I had are no longer functioning). The fish in it currently are 3 black neon tetras, 3 blood fin tetras, 1 threadfin rainbow, and a plecostomus(I will eventually trade him for a bristle nose). Needing to know if this combination of fish could cause any trouble or of if i need to get more black neons and blood fins to make them feel more comfortable. I would also like to know how many fish a 55 gallon could hold with fish of similar size to these.