I think you've got a few options when it comes to "cleaner" type creatures. If you had the three guppies, six neons, and three platies, you'd pretty much be fully stocked so there wouldn't really be much you could add other than maybe a couple of ghost shrimp.
If you skipped out on the platies, you could probably get away with a smaller plecostomus species. A single Clown pleco or bristlenose pleco could be possible. You'd definitely need to make sure you have driftwood in the tank, though, especially for the clown pleco, as they need to gnaw on it. They produce quite a bit of waste, though, so you'd have to really stay on top of your water changes. Whatever you do, just make sure you don't get a common plecostomus, as these guys reach 18" in length and need a huge tank.
You could also try a snail. Mystery snails make an interesting addition to your tank. They produce a decent amount of waste but not as much as even those smaller plecos.
Otos can be tough to keep. They really prefer to be in groups, which would push you well into "overstocked" territory, plus they can just be delicate. Some won't really accept prepared foods (like the algae wafers) and will really only eat algae growing in your tank, so they're prone to starvation.