Thanks for specifying! That always makes the job easier. I understand your situation with both a busy schedule and rough winters- Wisconsin is never an easy place in winter. If I understand correctly, you have the aggressive older gelding (the boss), the mare (his way of proving that he's top dog), and the Percheron who gets along with your horse. My advice then is this;
Coordinate with the older gelding's owner: this could be riding, riding with, even grooming this older gelding. The point is to understand how this horse ticks. If you've already got this horse down in and out, then that's half the battle.
Try to establish yourself as "godly" in the herd: If your only competition is an older gelding, you have a simple enough job. Just imagine multiplying him by three or four!
You are a human, remember that. It sounds stupid, but it's a simple thing that I've seen forgotten in people that were then hospitalized by a horse. You and your life are priority, you are essentially above the top dog. Don't let this make you overconfident, the horse can still do damage. But present yourself as important, confident, and aiming to do good in the herd and you will be regarded well. Know exactly what you want and be a leader in the group to get to that goal. Horses love leaders, and being one helps you out. Keep in mind that being positive and acknowledging positive behavior is better than punishing negative behavior.
If you have a dog, practice on them. This is where I get weird looks, but hear me out. Dogs and horses operate very similarly when it comes to displaying emotions, and they convey more than people often receive. Practice "tuning in" to your dog. Watch how they approach other dogs, situations, and people. This will translate into reactions within the herd, and can keep you safe. I hope this helped!