a pleco @_@ most plecos shouldnt even be in anything smaller than a 20 gallon tank i dont know much about loaches, but the small schooling fish machinasoul mentioned need a lot more space than they look like they need. you seem to realize that already though ^-^
i'd say snails or shrimp like everyone else has been telling you.
I'm really partial to rabbit snails. They need a male and a female to reproduce, so if you just got one, the chances of babies are smaller, though the two snails i bought both wound up being females who were already pregnant. They are live bearers, and though they can store the genetic material to make more baby snails, they only have one baby at a time, and like only one a month, so if you needed to give the babies away you'd have plenty of time. They grow about an inch a year, so they don't get too big too fast, and it takes forever for them to reach reproductive maturity. And they're cute. They have the cutest snail faces :)