The problem I’ve encountered with turnout in boarding situations is the turn out space is rarely large enough. Ten horses in a 30 acre pasture will not have the problems that ten horses in a three acre pasture will. With more space the horses can actually create small herds, get away from bullies, and have enough room to gallop. In a tighter area they can’t blow off steam as safely, many horses that get turned out will have that initial spaz of play time.
I had 3 geldings that had lived together for years. Usually a small pasture but sometimes stalls with adjoining paddocks. When I put them on summer board at a place that was about 55 acres with trees and a pond they stuck together for several weeks. By month two each gelding had formed a small band of mares and lived separate from their former stall mates. After about 3 1/2 months I brought them to the barn I had built. It had a ten acre hay pasture they could use in the fall and winter, otherwise they went from their stall/paddock into a two acre dry lot each day. By the end very first day they were back into their old roles.