The biggest problem in combining 7-14 day old kits is usually from them refusing to feed from the doe (since by this age, they know who their mother is) or else the doe refusing to feed them.
As far as weaned kits growing out, I've put all kinds of litters together.
I rarely see any turmoil or fighting of any kind.
I do wait until at least 6 weeks to wean though, and most often, closer to 8. I also tend to leave doelings in the cage with their mother and combine just the bucks from different litters together in a growout pen. I butcher 90% of what I produce though, and the bucks very rarely make it past 9 weeks old.
I have recently had an 11 week old buckling attack an 8 week old doeling, but, it is uncommon enough that is shocked me. He attacked me when I went to get the doeling out of his pen, and when I took him out for freezer camp, he screamed like he was dying whenever he was picked up. (He was held gently but firmly, I don't even scruff my rabbits.) Autopsy revealed no visible health issue.