Red rainbows (Irian) grow to 6", so a proper school is gonna eat up a lot of your tank space.
Boesemani grow to 3.5-4". Mine aren't a year old and two of my females are ~3" and they aren't showing their adult colors yet -- still greenish. I'm estimating their size on the low side, too. Three of my males are full colored, but need to darken, they're definitely close to 3". Another male is more yellow, and that could be a strain difference as he wasn't with my original school and I bought him separately along with my biggest female.
Both of those rainbows get very deep bodied, especially the Irian, so there's a fair amount of biomass there.
They'll be fine in a 55g, so long as you don't put any more mid-dwellers in there and don't go more than 6 Irians. With boes you could probably go 8 or so, I'd say 10 max if you keep the rest of the watercolum lightly stocked. Just remember that they are very active fish. Not hyper like danios, but always on the move and love to constantly swim in the current.
You also need to keep their water fresh. They'll do well with *no less than* 50% water changes weekly, and will do best with two or more 50% water changes per week. If you let the water get out of hand, they will fungus up very fast and can get very very sick.
Roan