BadHabit, you will get dominance displays between the same sexes ... bucklings generally earlier than doelings, but it will happen to all. That is the way of the animals.
What I have observed whether it is a mixed group or just a large/small litter:
1. About 3 months or so, the bucklings will start trying to mount each other to establish who is "top buck" ... one day it will be the largest buckling, the next day, could be the smallest one. There is no evidence of testicles yet, just the hormones getting going.
2. You may even see tufts of fur flying, but generally not much harmful biting, kicking, etc. However, I have seen a buckling run head-on into a wall and suffer a broken neck/back, so keep in mind that larger space available is good.
3. When I see this type of behavior (anywhere between 2-6 months), I seperate bucklings from doelings as it won't be long before the bucklings start developing ... and the doelings generally reach sexual maturity before the bucklings.
4. I use runout pens, and putting the doelings with the senior does is not a problem at all ... they will not challenge the dominate doe until after their first breeding/kindling if they do. Depends on how strong/dominate the top doe is. These heirarchical struggles can definitely result in fur flying and even nips to the face, ear, back, hips that can draw blood, but only show stock needs to be protected from this, if you know what I mean.
5. When bucklings are vying for dominance, placing them in with adult does can lessen or prevent this behavior for a while, but not for much longer. The adult does will not tolerate harrassment from bucklings. Placing the young bucklings in a cage/pen with an adult male will keep this behavior under control for a while longer, but bucks get much more serious about dominance at a point (and all breeds/rabbits are different, just watch carefully) so they will have to be housed individually ... I generally do this when the adult buck draws blood on a buckling for the first time, or the senior buck is really serious about chasing a buckling.
6. Another thing that I have found to help keep tempers from flaring, or dominance behavior to cause the rest of the cage/pen to "blow up," is space and things that can block the sight of another rabbit. Also, more rabbits can get along in a pen if they are only in it for a certain amount of time and then are returned to their individual cages at night.
So, for younger kits, go ahead and mix the litters ... they will have more playmates
Watch their behavior looking for signs that one is chasing or dominating the others. This is pretty much guaranteed to be a buck
If you have a large area for them to go into, with plenty of boxes, pet taxis, etc where the kits can go inside, behind, beside and not have to see whichever other one they are irritated with, that helps too. Run time with the adults is fine, especially if you start them young ... 6-8 weeks is a good time to introduce kits to a communal runout pen with different ages of kits and older does. But, supervision and attention to behavior and notice of changes is critical in successfully raising of any animals. Go slow, watch carefully, understand the different behaviors, and seperate any that are engaging in harrassing behavior longer than 20-30 minutes.