My understanding, and I won't claim to be anywhere near an expert on it, is that the fluctuation in stallion hormonally is much much smaller than that of a mare, and they generally speaking they will be open to mating near on any time if there is a mare in season presented - the basis of AI collection.
They absolutely can get along nicely in bachelor herds, but should you walk an in season mare past the bachelor herd to get to her field or on a hack it will kick off terribly, whereas should you walk a stallion near mares there will be interest but likely no fighting in any real ernest. In my experience anyway
Pure anecdata, but I know a few people who have kept stallions with each other or with geldings as company and not had any issues. For one of these people at least, there were mares around on the property and whilst the stallions/colts would be kept somewhat separately from the mares, they wouldn't have no exposure to in season mares. FWIW, the stallions never attempted to jump out to get to the mares either, but on one occasion, a pony mare did manage to get in with one of the stallions!
In the studies on American Mustangs, so effectively wild horses, they have found "bands" with more than one stallion present- I know some people don't like alpha/beta used in relation to horses, but I've seen studies that describe the band as having an alpha and a beta stallion- which also makes sense in relation to what I've seen with colts or stallions kept in company. As long as one is willing to be more submissive, they can get along very well!
Some of the studies I've read have also suggested that wild mares aren't as passive as once believed in terms of their mate choice- i.e. it's not just the case that the stallions fight and the winner gets the preferred mares by default, and some mares will actively refuse some stallions, or leave the band if they don't like (probably not quite the right word!) the current stallion.
I think, done correctly, a lot of stallions benefit from stable male company, and there's less risk of fighting involved than in e.g. unstable mixed herds which you find on lots of livery yards.
Not at all relevant to the thread, but I just think the UK especially has an odd attitude to stallions at times.