Bottom Feeder for a 10 gallon tank?

Snails are good bottom feeders that don't add any bioload to the tank. In a 10 gallon I'd be worried about adding any fish bottom feeders. Maybe Chinese algae eaters? They stay small (about two inches long) but are not very hardy (they only do well in a cycled tank).

The tank is too small territory wise for just about any bottom feeding fish, and plants alone will not allow you to overstock.

I think your best bet would be Ghost Shrimp - these can do well in subtropical temperatures. They get to about an inch, so are too large for your White Clouds to eat. And they are neat little bottom dwellers with a see-through body. They'll hang out mostly on the bottom and on the decor, they like surfaces. Ghost Shrimp, like all animals, do add to the bioload, but with these guys it won't be by much. Ghost Shrimp can be territorial if overcrowded, so don't get any more than 10 for your 10 gallon.

Another option would be a Mystery Snail. This particular type of Apple Snail will not bother your plants, and can live in temperatures from the mid 60's on up. Like most ornamental snails, they will require alkaline water with a pH above 7.0. Kept in acidic water, their shells will erode and they will eventually die. So only get one if you can provide alkaline conditions. These are omnivorous, and aren't really big on algae. So provide sinking omnivore foods for them. They will also need a calcium supplement, such as a small piece of unflavored cuttlebone. They'll also appreciate blanched veggies like Zucchini as treats. Mystery Snails will get about the size of a golf ball, but produce a lot more poop than you would expect - these have a fairly large bioload, even when compared to fish, so keep that in mind - you shouldn't get more than 1 with your White Clouds already in there.

Unfortunately in a 10 gallon you are very limited. A bristlenose pleco can do ok in cooler water, but need at min1mum a 20 gallon, ideally a 30. Cories need at least 75 degrees, and loaches as well. I can't think of any bottom dwelling fish that would do well in this size and not have the tank heated. Sorry.

The only cold water bottom feeders I can think of are Hillstream loaches and snails.

Maybe a few Corydoras? I've gotten a few corys in my years of fishkeeping that didn't get any larger than 2 inches.

Snails are about the only one you can use. The temps for tropical fish should be at 75-78 degrees - any catfish or cleaners need that temp too.