Sounds like you have a big bioload. The best thing you can do to prevent algea is to limit feeding and do lots of water changes. Plecos are good but they only eat certain kinds of algea. You obviously know they wont eat cyano, but they also (from my experiance) dont eat hair algea. They also can damage thin leaved plants. I would take him back and get a bunch of octocinculus (Sp.?) they are neat little algea eaters. I know its tempting to get every fish out there but I found after lots of tanks with every kind of fish that species only tanks are much nicer.
Ok I kinda went off on a tangent there but good luck and dont get frustrated.
If you are doing a planted tank I would check out ...
http://www.thekrib.com/Plants/
Its a pretty old school site but has lots of really good threads.
Good Luck,
Jon