I'd bump the school to at least 11. They are not small tetras, so I'd probably keep them in a larger tank too.
Even with a larger school there will still be rough housing, a bit like teenage boys. But, it gets spread more evenly than when 4 fish decide the 5th fish is it.
Cories are social by nature and do better in groups of 6+ individuals. What kind of plants do you have? Are there enough breaks in line of sight between the fish if they want to get away? What type of catfish is Vanilla? Are there enough hiding spots for it?