You can get a 55, but the plecos would benefit more from a bigger footprint. So a 40 would be better. If a 75 is still within "smaller tank" for you then that would be even better. The 55 is too narrow, and that extra height won't benefit the plecos at all.
The bigger footprint will allow plecos to be kept with less territorial disputes since there is more land for them to be on. I have quite a few in my 40, more than 5 and they are alot happier than they were in my 55 that also had lots of wood and rocks