I have six horses and have always had a problem making sure everyone gets enough of the right food when I grain them. I don't have a barn (yet), and pulling the horses out to feed them individually doesn't work out all that well, since Bandit and Dante won't eat while tied unless I hold the bucket up to them and 2-Pak will bolt his food and get pissed because everyone else has food and he doesn't.
Feed time now pretty much goes like this: Bandit and Dante's rations are combined, because they always share a bucket; Magic and Tanner's rations are combined because they share a bucket; Ruby and 2-Pak get individual buckets.
I'll dump half of 2-Pak's ration into his feed bin, then Bandit/Dante's, Ruby's and Tanner/Magic's, in that order. By the time I've passed out everyone else's rations, 2-Pak is polishing off his half and I'll give him his other half.
Usually, at some point during this, Bandit and Dante will start poking their noses into everyone else's buckets before actually finishing their own, Ruby will decide that someone else is eating too close to her, and she has to stop ro run them off and steal their food, and if at any time 2-Pak discovers that his bucket is empty, he'll either share with Bandit and Dante, or run off any of the mares for their food.
If it helps any, here's a rough layout of the feeding area:
(the unmarked black mass is the garage... no horses allowed.