most common varieties of pleco will eat algae as youngsters. Many of them develop more carnivorous tastes as they mature. Clown plecos--it really just depends on the individual. Clowns are better algae eaters than most of the fancies, and definitely better than common plecostomus, but they have been known to occasionally tear up plants.
They are a good size for tanks down to about 20gal, and fairly easy to find, so if you like the looks just remember to keep him well fed once the algae is under control. Chances are there won't be a problem.