Pleco's have a large bio-load for their size, but the smaller plecs are great in a tank that size. I highly recommend pitbull pleco's. They are the smallest I've kept, right around 3" with a small body mass so bio-load isn't too bad. They have a beautiful iridescense to them, in addition to being just darn cute. They are effecient algae eaters as well. They are social and shouldn't be kept alone, I would recommend stocking atleast 3.
Most hillstream loaches are awesome algae eaters and their bio-load is pretty small. They need cooler temps and good oxygenation in their water, so they won't fit in just any community. I don't know what other fish you'll be stocking so not sure if these would work for you.
Oto's are highly effecient algae eaters. They need to be added to well established tanks. If your tank has been running for atleast 6 months, you could easily add 6 oto cats.
Or there is always nerite snails. Incredibly effecient algae eaters. They'll detail your decor and plants as well which is great.