I would go with a single serra species. Reds are really freacking boring when they are solo. I have one solo rb that is waiting for the rest of my shoal to grow out to add him. And he is a dud... The group is more much interesting to watch and keep.
A 55 is pushing it also with most species except maybe sanchezi's or macs. I have a 7 inch elong in my 55 and trying to make room to get him in a 75 or larger soon. I feel bad keeping him in the 55 because of how small the depth is (12 inches).
Its also too small for a shoal of rbps or caribes, and definitely piraya.
A juvy rhom you could do for a while, but will need plans to upgrade down the line.
I would go with a small elong for it and give him open space to swim and skip the divider and a red.
A 55 is pushing it also with most species except maybe sanchezi's or macs. I have a 7 inch elong in my 55 and trying to make room to get him in a 75 or larger soon. I feel bad keeping him in the 55 because of how small the depth is (12 inches).
Its also too small for a shoal of rbps or caribes, and definitely piraya.
A juvy rhom you could do for a while, but will need plans to upgrade down the line.
I would go with a small elong for it and give him open space to swim and skip the divider and a red.