Kuhlis aren't aggressive. They're generally quite shy unless you have a big group of them. They're also very skinny and quite low bioload despite their 4" length. If this is for the 55 you were talking about in your other thread, the convict would probably make spaghetti of them, but if you decide to move that to the other tank, I'd suggest a group of 8+ kuhlis if you get them.
Corydoras are always great, IMO there's no better option for a community bottom feeder. Peaceful, playful, social, and unlike loaches won't eat shrimp or snails if you have any. They are very curious and pretty dumb, which can be a bad mix if there's territorial bottom dwellers around. Mine get chased by my petricolas, bristlenose, and keyholes occasionally, but have never been hurt by it, or even deterred from snooping around other fish's lairs.
There's plenty of nice smaller loaches. Zebra loaches, weather loaches, polka dot/burmese border loaches are attractive and still smaller than clowns. Synodontis nigriventris (upside down catfish) and petricola both stay under 4". USD catfish look sa lot like a cory, but has the barbels and behavior of a syno.