It seems everyone else has already answered your first question: Yes, the hair should grow back, so long as normal skin is grown back in the place of scar tissue, which seems to typically happen. I do not know why sometimes a body regrows normal skin over an injury and sometimes grows scar tissue. I think, but am not one hundred percent certain, that if the other horses aren't actually tearing off all the skin layers (which would leave a very bloody mess with visible muscle wall and/or bone, depending on location) and so long as it's only the upper layer of skin that is removed, it should grow back normally. In humans, what we perceive as scar tissue, over very shallow wounds, such as scrapes or small burns, is actually normal skin tissue, it just grows back sometimes with a different skin color, sometimes darker, sometimes lighter. Actual scar tissue is typically raised off the skin and is often times almost opalescent in the way that it reflects light. Scar tissue does not grow hair, because there are no hair follicles, because the skin was destroyed. Therefore, so long as they are just skin wounds, they should heal up just fine, though there might be spots here and there that do not grow back as well. Also, sometimes scar tissue will never go away, and sometimes the skin will slowly regrow and remove the scar tissue. Sometimes the skin will just grow over a scar, and the scar will be under the horses skin, as my horse has on his hip, where he was injured in the trailer ride on his way to my house. It healed and the skin healed over it and there is no problem, just looks a little funny!
Second question: While it is normal for horses to play fight and normal for them to establish "who's boss," sometimes they just flat out do not like each other. This can result in a lot of fighting and a lot of damage. When I moved Siaga to the stable he is currently in, I made sure that there was something in place for this. The lady told me "No horses are allowed to fight. Play and the dominant issue is one thing, those are normal. If they just don't get along at all, they won't be turned out together. I won't even keep them in stalls that are side by side or across the isle way from each other."
As he is your horse, you have every right to request that he not be turned out with the 2 year olds because of the damage they are doing to him.