I don't use anything on my stallions other than to increase their grain and protein levels to accommodate their more energetic actions.
And check their vitamin levels. I have had the odd one drop down in Vitamin E and so I will give that or if one is slightly too aggressive (Usually a young stallion learning I will give vitamin B for its calming bonus)
I haven't found any supplements to be necessary here. My stallions are on a balanced diet year round and maintain their weight well. They generally live with their mares and they seem to be less stressed. The boys are pretty fat and sassy!
I used to stand Appaloosa and QH stallions, not sure if you do things different with Mini's, but the only supplement I gave them was kelp meal from a local company. I would up their grain a little but that was it. I had a 100% conception rate and they never lost condition, so I guess they did fine with it.
I feed mine as much hay as they can clean up and if they can't keep weight on that then I add grain to their diet. If that still doesn't keep the weight on them at certain times of year
I add some fat (corn oil) to their grain. Be careful with adding selenium - you only need to add it if it isn't already in their diet in sufficient quantities.