Clown loaches can get to a foot long, red tail sharks only get to 6". The sharks are also very aggressive as they mature, and one would need to be the only bottom feeder in a 20g. In a very large tank, they would probably be okay together, but a 20g isn't room for clown loaches anyways. If you get a redtailled shark, you'll have a semi-aggressive aquarium started, so keep that in mind as you're planning the rest of the fish.
Edit, wow, I can't read, I thought that said 20g. In a 250g, I don't think you'd have a problem with both. You might want to either add the clowns first, or make sure they are larger than the shark, or both. The clowns grow very slowly, and need to be in groups of at least 3. I would still stick with only 1 shark, but in that large a tank you might be able to get away with 2. Make sure to have lots of caves around for them to hide in.