All mine are in together form September until the end of March or possibly April (I have to keep an eye on Carlos, he will start fence running if he gets stressed, and also attempts to keep the others off the fenceline.) There is a bit of squealing when they first go out but they know each other well enough now so they settle quickly. The trick is mainly to give them enough room and make sure that Carlos' mares are a field away to start with. I also have a double hot wire between the boys filed and the colts next door- it is not really necessary but it is just peace of mind!
I take Carlos out from his mares on the first of September- there is no way of saying if a mare will lose the foal early on, even if I tested, which I don't, and I don't want any "Oops" foals in the middle of winter- I learned that lesson the hard way and one 27th Dec foal is quite enough for this lifetime.
Once in a lifetime you find a horse you cannot do this with, at all, no way, not going to happen! Rabbit is my "loner" no way will he mix in or play nice so he has his own paddock. Even in summer he has to be kept seperate, although he does have his round pen in the mares field!