I'll be getting a 4 month old weanling in September, so we're almost in the same situation. I have a very maternal maiden mare, who tried to steal another mare's baby, and a welsh pony gelding. The gelding may be proud cut, because he acts real studdy with other mares besides mine and will try to mount them. But the poor thing can't reach! So, we have a very large foaling stall that's not being used, so we're going to put the weanling in the foaling stall for a few days. My two horses are the only ones on the 10 acres, and they have 24/7 turnout so they can go check him out whenever they want without being able to harm him. The colt that my mare tried to steal stayed there for a year, and my pony had no problem with him even though he'd never been gelded. It's all about a slow introduction and taking the right precautions.
As for feed, my foal will be on Nutrena Safechoice. I've never used it, but my breeder has, and everyone who's used it has great things to say about it.