Stallions are very persistent. Unless you want some funny paint/mini babies, keep from away from the mares. Some gelding friends would be nice though. My BO's mini stallion lives with a mini gelding. The stally's a bit of a bully, but, otherwise, they get along just fine.
Have to agree, a stally should never be turned out with mares unless you are expecting and wanting him to cover them. I ended up with a pregnant Belgian mare because her old owner had turned her out with a QH stud because "He couldn't possibly breed something that much bigger than him".
My friend has her mini stallion turned out with the other full size mares and geldings. I've known her for 5 years & no accidental breedings yet. Although he does try.
The only problems she has had with the mini being out with the other horses is her draft gelding really bullies him, & the mini doesn't always get enough to eat. She puts him in a stall ever so often so he can get extra food.
She also has plenty of room for everyone to get away from each other.
The mini actually kind of stays off to himself unless he is pestering a mare.
He is also protective of the herd. If a strange horse gets to close to the gate, he charges the fence.