pleco per gallon rule?

Do any of you use pleco's?

Is there like a pleco per gallon rule or something?

For bottom feeders I have two Cory's and one 6" pleco in a 55g tank.
The Cory's only eat food off the bottom and the pleco just hangs out
HA! making short work of diatoms(SP?), a little algae and some stuff
I'd rather not talk about.

Do you think I should get another pleco or some other fish to help
clean up? My only complaint with the pleco is they tend to get prety
big over a couple years.