Adding more fish to a tank that is pretty nearly stocked is a bad idea. Plecos and loches get fairly big, have different temperature requirements and have large bioloads.
I'd either get assassin snails or put a plastic container with small holes (to allow water to pass through and not the snails) then sink the bowl and place a piece of lettuce or other vegetable in it and leave it over night, or a few nights. The snails will go the the vegetable and when you pull the bowl up the water empties out through the holes, leaving you with snails in it.