I have two roosters, now one year old, and I have been through this, which they started at about this same age. I ultimately decided to let them work it out- they're both standard heavy breed roosters, equal in both size and determination (which is the root of the problem, if one would just go submissive we wouldn't have a problem) so I don't worry much that one will fatally harm the other, and like you I don't really have anywhere to house them separately yet. I only separate them if one seems at a disadvantage (one time one had a swollen eye). To separate them I simply let one out of the run and keep the other inside during the day (they still sleep in the same coop). They have fought maybe four times, with the fights usually dragging out over a few days. There have been a few flesh wounds and their combs aren't as pretty as they used to be, and I did just notice that one (the now dominant rooster) is missing a toenail, but they have never seriously injured each other. And they are both great roosters- they tidbit, do the wing dance (apparently these two traits have been accidentally bred out of some breeds), give warnings of danger (it's usually a hawk, this is absolutely vital to my flock), try (and often fail) to keep the flock together in the yard, let the ladies eat first. Both are good in size and conformation, although I just figured out one is a mutt (I was told by the breeder he was a Wyandotte). BUT part of the reason I can tolerate it for now, as long as they aren't seriously hurting each other is 1) they free range a lot, which means they can stay out of each others' way, and 2) I'm pretty sure both are going to be coq a vin come fall. Not my favorite thing to think about, but it's the way I've chosen to run my flock, and my plan is to only keep a Buckeye rooster and no others over winter. Are they in the coop only, or do they free range? In a coop that big with that many chickens you can easily and cheaply partition off a small area if you need to. I don't know what you have on hand or can find people giving away, but you could use pallets (a local implement dealer here has a huge drift of them behind their building, and I've heard the people at Ace say they just get rid of them), scrap lumber, poultry netting... be creative and check around for free stuff people just want to get rid of.
So far as introducing the new members of the flock, it depends kind of. Your flock isn't so large that the new chickens are going to be totally ganged up on, but there is a pretty big difference size wise between 1 month old and 3 month old chickens. Your best bet would be to keep the new ones isolated for a week (good biosecurity practices) then have them so they are separate but can see each other for a few days- you said you had the ones in a dog kennel right now, once the isolation period is up just put the dog kennel in the coop for a few days. Then, at night when it's dark, put the new chickens in with the older ones. Then watch them. If the older ones noticably pick on the younger ones (and it's normal for them to peck a few times to establish the order, but they shouldn't go much beyond that) you may need to remove them until they are bigger. But it's usually easier the younger they are as well. I just put a batch of 2 week olds in with a batch of 4 and 5 week olds and had zero issues. But that can have a lot to do with the temperaments of the chickens involved as well. Just watch is the most important part. Plan to spend a few hours the day they will first be together just watching them- clear your schedule, so to speak:)