I've kept common plecos in 30g tanks before. You just need to have an adoption procedure in mind for when they outgrow it in a few years. Common plecos are about $2-$5 a pop and have lots of personality (if you can get them to come out during the day). I just traded back to a baby one in my tank last month. I just put him in a little carrier with tank water and a bit of substrate and dropped him off at the LFS once he hit 5-6" and they traded me a new baby one for him. The baby one is about an inch and a half long and just loves all the treats I had for the bigger one.
Good deal for the LFS, since babies cost 2 bucks, but medium adults go for $10-$20 depending on breed.
Incidentally, as they get larger, they do dig a bit in the corners. I ended up putting heavy deco items in the corners to keep the sand from being moved so much while he was mucking about at night.