Leopard ctenopoma - predator and not a bottom feeder, so that's probably out. Awesome looking fish, but they hide in plants and such and ambush fish.
Pictus cat - is a very active bottom feeder, but will eat small fish that can fit in its mouth. I believe they get to 5-6" or so. Gets too big/active for a 16g or smaller, and will eat lots of guppies and babies, so it's probably a no too.
Zebra botia - I'm assuming it's like most loaches (specifically yoyo loaches), in which case it would probably be fine assuming you mean for the 75g. They need groups of at least 3-4 to be happy. If it's the type I'm thinking of, they are very pretty fish. Fairly active and playful but not aggressive (may eat baby guppies since they're so small, and will also eat snails).
Paleatus cory (or any type of cory) - good little active bottom feeders. They also need a group, at least 3-4. If you are indeed thinking of the 75g, I would personally go for a much bigger group, maybe 8 or more. They are so cool, and like to follow each other around. They should leave even baby guppies alone. Get up to 2-3", so not too big. Lots of fun to watch. They'll even go up and down the plants looking for food particles.