Several things come to mind while reading your post. First is that if you don't know about halter
he is my first halter horse so I don't know a whole lot about the event
then, before you do anything, you should go to some local shows and see how and why the judges place the horses. Ask a lot of questions and ask the winners what they do to prepare their horses. If you don't understand the event, how do you expect to compete? There is much
more to halter then showing up with a well conditioned horse.
As for supplements to increase muscle .... those are called steroids and a no-no for anything. If your horse needs more energy, look to your feeding program. The use of alfalfa can be a big help. If you can't get alfalfa hay, you can always get pellets or cubes - just be careful with it.
As for ponying, the fact that your horse doesn't like it so you don't do it, means he is lacking in his training. It isn't a matter of whether he likes it or not, he does it because that is what he is supposed to do. The fact that you have a gaited horse and your colt can't keep up at the trot, just means that you should slow down to a walk.
The long and the short is that exercise - hills, a good diet, and more exercise is what you need to do. There are no supplements that you can give your horse to take the place of the proper conditioning and the proper knowledge.
When I started team penning, many years ago, I went to quite a few shows and watched, and asked questions, and watched some more. I took what I learned from attending the shows and trained my horse (he had the proper breeding but not the right training). I then took my boy to a few shows just to acclimate him to the event before ever attempting to put him in the arena. We penned for a good number of years and won a lot of money. It all came from experience and the proper preparation.