I agree with what Daniel has said, it is difficult to recommend a plec without knowing what else will be in the tank.
however, just throwing out ideas...I'd go with 1 or 2 bristlenose. they won't grow as large and won't make near the mess that a 12" or larger common will make and they should do just as good a job at keeping the algae clear.
I would say a bn pleco or a rubber lip pleco. They both stay small. And like everyone else said the common plecos make more of a mess then they clean up.
Depending on what is going to be in there, maybe a chinese algae eater would work good.