OP: I noticed you have not said anything since your original post. As far as the "fighting" goes I would try to separate them at feedings and if feeding hay throw an extra flake. We just introduced a 4th horse to the pasture at the barn where I keep my horse and we feed hay. at first the new horse and my horse did not get along at all after about 2 mins my horse was removed and the other 3 got along fine. I went out the next day and introduced the new horse and mine again. There was kicking and biting and pinning of ears and some fence rattling but no one got hurt. I watched them the whole time, no halters, when things settled down I let the other 2 horses in with them. My horse still chased the Filly when she came close and challenged his position in the hierarchy. when Things had settled to a level where I was comfortable they would not injure each other I fed them. Each got a flake of hay separated by a few yards and because my gelding kept chasing her off I walked another flake to the far end of the pasture where my gelding would let her eat. It has been about a week and the fighting has stopped and they can be fed near each other now (I always separate feed by a few yards. If they are on special diets I would separate them completely for feeding. If they are just playing I would leave them be, they won't usually injure each other too bad if they are just playing.
As far as the halter issue. I prefer my horse not to wear one when not necessary but if you feel the need to leave them on I hope they break under stress becouse they can be dangerous if they don't. Please let us know how things are going. I would love to know what has worked for you for this problem.
"It's a lot like nuts and bolts - if the rider's nuts, the horse bolts!" ~Nicholas Evans