Mine are all turned out together (2 mares, 1 gelding). They come into their own seperate stalls to eat in the morning and evening. I whole heartedly believe horses do better with maximized turnout, with other horses. They are herd animals and are happier, thus healthier in that type of setting. 

   Of course if you have one that is a trouble maker, that changes things a bit. But mine do very well this way.

As long as they get along I will turn them out together. I do currently have my mare separated as she is close to foaling and my one gelding does not get along with her AT ALL. She is lower on the pecking order though. 

My 3 get turned out together in the pasture during the day .. put up in separate stalls in the evening and at feed time. I keep them out of the barn during the day... doors closed so they cant get in. I've had a situation where the dominant horse has cornered the low man inside the stall and beat him up pretty bad. 

We only have 2 acres right now, and we have 6 horses turned out together in a 1 acre dry lot. They have their herd hierarchy worked out and get along fine. I have a 2 others I can turn out with them as well; one's getting a training tune up right now so I have her in a pen of her own while I'm working with her, and the other is pulling trail horse duty at my mom's house. So altogether we have 6 geldings and 2 mares that can cohabitate a fairly small area.

You have to make sure it's an open space with no small corners for subordinate horses to get trapped in. Mangers need to be spaced well apart, with a couple more than the number of horses (I have 8 mangers for the 6 horses turned out together right now). The water trough needs to be in an open area, not a corner, and preferably on the opposite side of the pen from the feed. 2 water troughs is even better, then a dominant horse can't guard and control the only water source.

There are some horses that just don't socialize well. They are too aggressive and spend 24/7 harrassing the others, or they are too submissive and get beat up or starved out. Or they might be older or hard keepers and need time to eat alone. But I like to keep them in groups if personalities and paddock layout allows it.

When/if you try to introduce them to herd living, it helps if you do it in buddy pairs. So take a couple that get along and turn them together for a few weeks, until they really buddy up to each other. Then introduce them to another pair, and so on. That helps keep any one horse from getting singled out and picked on.

I turn all mine out together... I had 4 on 10 acres, now 7 on 26... mares and geldings...