I would still not subject a fish whose natural environment is a fast flowing river to a 5 gallon tank. I can't recall the exact figures right now, but I believe the majority of plecos bought and sold in the pet trade are wild caught (some of the more popular varieties are farmed, but a lot are taken from the wild). Sure, no tank could compare to the space plecos would have in, say, the Amazon, but at least they should be provided with enough room to explore, exercise, and develop properly as juveniles (filters do not take care of growth hormones, btw).
Think about it this way: would you stuff a child in a large cardboard box? Say it's a nice big comfy box... There is enough room for the child to move around and even fit a few toys and you can keep him well fed, clean, and entertained until he or she grows to adulthood. Would you? Probably not, I hope. Small children need the space for healthy, normal development. It would not be right to keep them in a box until they physically outgrow the box even if you can provide them with everything they need to survive.
I try to apply that same principle to the fish I keep.