So much depends on the individual stallion. Our senior stallion had NEVER been with another horse (except to breed) and was in his 20s when we acquired him. He became noticeably stiffer in the winter in his stall so we made the decision to turn him out into a large, centrally located private pasture which was how he had been kept at his previous farm. No go. He got down into the corner closest to the barn and stood there, which entirely defeated the purpose of turning him out. Finally we took my 2+ year old colt and turned him out with the stud. After the requisite running around and checking out of undercarriages they settled down and were contented pasturemates for the better part of the year until we took the colt in to start. We replaced him with 2 weanlings and were astonished to see the senior stallion become a doting ‘father’ (weanlings were unrelated to him), carefully rounding up the colts and ‘putting them to bed’ in the run-in shed when the weather was bad (I have pictures of him standing and laying in the shed with them, blocking the door while it was snowing). Since then he’s had a number of babysitting jobs, including our current National Champion 3 yo colt. He is a big stallion, and takes no guff from the colts, and everyone pretty quickly learns manners, but having the youngsters around keeps him moving and flexible. Stallion is supremely happy with this arrangement and the only injuries we’ve ever seen were to blankets and halters, which apparently becomes some kind of frat house hazing ritual (dude – where’d you get the lame-o halter and how fast can I get your blanket off!).
Having said all that I have another stud that I would never dream of putting out with anything other than the hard-as-nails little gelding who kicked his arse as a colt. That stallion is very studdy and will. not. quit. except with the gelding. 
But that is his personality and he is like that in all aspects, so its hardly a surprise.
One odd footnote to add onto something other posters have mentioned. The 2+ year old colt I put in with the senior stallion initially was subsequently gelded after he was saddle-broke. At one point I attempted to reintroduce him to the herd (senior stallion and 2 weanlings) and his former pal chased him all around the field with bared teeth. Not sure if it was something to do with the gelding process, or if senior stallion was protecting the weanlings from someone he no longer recognized as part of the herd. We never had problems taking one weanling out and introducing another/different one, so I have to think it had to do with the gelding/hormone shift/smell change. No mares were involved and it was late fall when we attempted this, so spring was NOT in the air. It’s the only time I actually saw that stallion get aggressive with another horse.
