Hello,
Based on my limited experience, it is down to individuals. Right now I have 3 CB specimens and 8 WC specimens ranging from 5" - 19" all in a single aquarium.
The WC IME are less prone to cannibalism, but are right through bullies. My newest addition, a 17" PBB bullied the "quiet alpha" 19" PBB including the rest of all my fishes. I have a 6" lap in there as well who got it's tailed really messed up, but no attempts to eat it whatsoever, even if starved.
You get WCs with an appetite, and some who love to go on hunger strikes. I realized that the more polys I introduced, the more they are stimulated to feed. But some just don't care and will eat literally anything.
CBs on the other hand will eat anything, including pellets. I've recently had success with my WC ornate in getting her to eat massivore pellets, but no luck so far with the others.
In terms of aggression, the more polys you have, the aggression spreads out. Said bully PBB is a bit lax now towards everyone, but definitely is a total d****e to a single eastern lapradei I have.
I don't see you having issues with UJs as they are quite peaceful compared to LJs. My WC retro is one hell of an eater and gets along very well with everyone except for the bully. She's the longest UJ I have at 11".