Hi Snake!
What do the lps have available? I would steer clear of lps pandas, although a large group (10) in a species only tank might do well for a few years. C. sterbai are an active and medium sized Cory. If you want to try breeding, C. aeneus (bronze) are always ready to oblige. C. paleatus (peppers) are also a popular and hardy Cory that is easy to breed. There are several Cory species that show up in the lps that are great little guys. They are usually mislabeled so it is hard to go by the labels. In a 20 usg you can keep a nice shoal of Corys. I would say up to 10, but it would be wise not to add that many at once. I would say a nice group of 5 or 6 smaller, younger cats.
Oh! and Emerald/Green Catfish/Corys (Brochis splendens) (they go by many names) are also a nice fish. They are larger when grown, so 5 would be good.