Maybe 3 or 4 otocinclus, or a single dwarf pleco, like a bristlenose or a rubberlip/rubbernose/bulldog. Also clown plecos don't get big, either.
The otocinclus I have had have been fabulous with diatoms (brown algae) but did not handle the green stuff. Don't know if that is always the case, or if mine were unusual in this regard.
You could also try to find a true SAE