At, that's a shame. My turquoise is almost 5 years old and still only a little over 3 inches, so I assumed that's the max size he would reach in my tank. I unfortunately cannot vouch for how well he's been fed for some time though.
If I stuck with the Maccullochi, how large of a school would fit in the 55?
Threadfin rainbowfish also exist, and you can get a massive school of those.
As for the stocking plan, what type of filtration do you have on the tank? It seems like you're running the tank temperature low due to the bronze corys and dojo loaches (both of which appreciate not super warm water). What's your water change schedule like? Plant cover?
Regarding the Macculloch's, since they're several years old, have you considered breeding them in order to bring up the school size, rather than acquiring new fish?
I will be running an external canister filter with ~525gph that should be arriving soon. The water temperature is around 70-72F.
Since this tank was just re-setup this week, I don't have a set water change schedule yet. I think I'll be trying to follow Tom Barr's Non Co2 planted tank method which limits the amount of water changes, but this is purely just an experiment. The idea of a low-tech tank is attractive. But if things don't work out I'm used to doing large weekly water changes and have no problem doing them.
I had to go and buy new plants in the past few days so they are still acclimating to my tank. My goal is to have it densely planted but right now it's lightly planted at best, although I did try and plant heavy to start. I don't plan on running Co2, but I have a pressurized setup if I need to switch over for whatever reason.
Would the threadfin be compatible with the Maccullochi? A nice big school of those would look great but I'd worry about nipping of their fins, and would my filter flow be too fast for them? And breeding the Maccullochi is an interesting idea I hadn't thought of. I might look into that and see if it's something I could do.