Don't assume anything. It may be fine or it may not be, and things can happen so quickly.
As several have mentioned, every horse is different. I know of a number of stallions who are excellent babysitters, but I would NEVER trust either of my geldings with a foal, a weanling, or any horse that is less than completely self-confident. We waited until Scarlet was big enough and "mare enough" to stand up to Mingus and Thelonius before putting them together. They are sweet, gentle lovekins around their humans, but they believe strongly in the pecking order around horses and other animals.
Just to show how much you cannot predict, I thought gentle Thelonius would be a great companion when Flash was a weanling, since he was always so meek. Evidently he saw this as his chance to be dominant for once and tried to run the little guy. Once Flash was a bit older they became great buds, but nor unril he could stick up for himself.