I would recommend steering clear of molasses if you think your horse gets hot off her feed. If you're looking to simplify your horse's diet and help her gain weight at the same time, you should look into Triple Crown's senior feed. Even if your horse is not old, senior feed is more palatable and digestible and the Triple Crown Senior seems to have a higher fat and fiber content than other brands.
Our horses get Southern States Solutions. This feed is also fairly high in fat and fiber and has done a wonderful job of keeping our thoroughbreds, and one senior thoroughbred-type appaloosa gelding in great body condition, without making them hot, and without feeding them large volumes. They're not getting good grazing right now either because of the weather. We're buried under more than a foot of snow and temperatures have been in the single digits (Fahrenheit). The horses get hay, but nothing over the top in quality and they're still in good shape and keeping warm with their current feeding regimen.
If you're really stumped, perhaps you should talk to your vet or an equine nutritionist.