Honestly, I don't think it would be wise to be adding fish right now. First you need to get your aquarium healthy, and prefferably get some more filtration on there. As far as the plecostomus goes, I'd strongly recommend you don't get one. Go for a chinese algae eater if you want one, or a few otocinclus, but pleco's get huge.... literally hitting 18-24" in length, and sometimes in shorter amounts of time than predicted. Beyond that, they need a much more varied diet than algae (and rarely eat algae once they begin maturing) including zucchini, romaine lettuce, squash, and eventually even some fish meat.