One of my four horses is bonafide metabolic and a second one is trying to head there.
Feeding my metabolic horse is a complicated matter so to keep my feeding life simple, I switched EVERYONE to pelleted rice bran.
All of them also get Omega-3 horseshine, a pelleted vitamin/mineral supplement, and a splash of warm canola oil more for the flavor than anything.
However, that warm canola oil doesn't stay warm long in the cold weather and two of my horses won't eat their feed if I've added too much canola oil and it's gone cold and yukky.
My horses are on 12+ acres of pretty good pasture, they each get a flake or so of grass hay when they come in at night, so they don't get much feed.
The metabolic horse gets the most pelleted rice bran, being fed 12 ounces 2X/day this time of year; I cut that back to 8 ounces 2X/day in the warmer months. Everyone is maintaining their weight.
Pelleted rice bran is 20% fat, so it might work for all of your horses, once you figure out who needs the most and who needs the least.
I prefer the pellets because it's a more enjoyable volume of feed for everyone.
Hope this helps some