Ok, measuring grain in cups is a big pet peeve of mine. You need to weigh it(you can cheat by using a 1lb coffee can). The only people in the world who measure horse feed in cups are miniature horse people!! Its like measuring at the last hair of the mane. LOL! Right now I have an A mini filly and Classic shetland filly in a stall together. They each get a pound of grain plus 1/2 pound wet beet plup. I realize that they both don't eat equal amounts and 1 may eat more than the other, but that's what is offered to them. If they clean it all up, I give them 1/2 lb more until they no longer finish it. If they don't finish it, I hold what I'm giving them because eventually they'll hit a growth spurt and start finishing that amount. Make sense?
On exercise, there's a reason a lot of cattle operations use feed lots instead of free rangeing their cattle. It increases feed consumption and decreases calorie burn off which increases muscle mass. Same thing in horses. If you are going to show them, they really need to be stalled for the most part of the day. This way they put on pounds and take full advantage of any turnout they get. Our weanlings are stalled 24x7 if they are to be shown and turned out a few hours each day. All that pent up energy provides plenty of encouragement to exercise when turned out. Plus lets of practice leading back and forth!
Another piece of advice, if you are serious about showing weanlings, especially in futurity or sweepstakes classes, I highly recommend that you do not wean them. Staying on momma keeps that nice topline and slim belly. If you can't haul the mare, you can always wean right when you load everyone up for the show and leave mom behind.
Edited to add: I prefer a pelleted grain over sweet feet to feed show horses. Just my preference. Sweet feed attracts flies.