Neither clown loaches or cories are aggressive fish normally. I HAVE seen clown loaches pick on a sick/dying fish, though, and as their name implies, they sometimes playfully chase other fish around. But cories are kind of "armor plated" and unless they lose their fins, are usually OK. Not much messes with clown loaches, they're too fast.
The only thing to keep in mind is you have a fairly small tank, and clown loaches do best in schools of at least 3 (5 is better.) Also, cories like to have at least one playmate of their own kind. That's pushing it for a 20 gallon tank.