They're getting the idea...or an idea...somethings going on here...what?!? Where's the food??? Wonder what they'll think when you go out and release them...and keep up the game? LOL
It honestly does not sound like that much of a problem -- horses just DO chase and threaten and kick at each other. Chances are very good that they will eventually work out the pecking order, so to speak, and settle down and get along better.
In the meantime you'd be best off respecting the hierarchy that already exists -- this means putting the mare in FIRST and feeding her FIRST. Both horses know that's the way it ought to be, and things will go a lot smoother if you don't try to disrupt their little herd structure. (You CANNOT reverse herd hierarchies by handling horses in opposite order -- it just creates chaos and resentment and problems).
Mind, she needs to be civil and safe around you. If you are unsure of her manners when both you and the gelding are around her, I'd carry a longe whip or long dressage whip with you for a while. And she should only be allowed to be haltered, brought in and fed if she is CIVILIZED about it. But, always do her first, then the gelding. After feeding, turn her out first unless she lurks at the gate waiting to 'get him'... if she does that, it's safest to put the gelding out first.
Do you by any chance have enough stalls that they don't have to be in adjacent ones? Or if they do, put their feed at opposite corners of opposite sides of their stalls, so they are not right on the other side of the shared wall from each other.
Generally though things will settle down once they've got things worked out better between them -- mainly you just have to keep anybody from getting hurt til that happens.
I am sure they'll work their issues out, it sounds like they are already on their way. You just need to stay safe when on the ground and they are loose until one of them concedes the other IS the boss. It sounds like you are doing a great job already.
The mare may regress each time she cycles into heat, but the gelding will learn to avoid her.
You're lucky the gelding you got didn't have mare issues. My appaloosa is a FOOL around mares.
I agree with the last couple of posts...it hasn't been very long and they will work the pecking order out. It's natural to see more dominance displays during feeding time. Being the herd leader gives the boss the right to eat first. There a few exceptions in my book...I don't tolerate aggression towards people during feeding period. And occasionally at work we'll get a mare in that doesn't do well being fed outside in a herd situation, before they start dropping weight we try a different herd and worst case scenario they get fed in a small paddock.
Something you wrote that I was happy to read was..
None of the aggression is aimed at people, but if someone got in the way....
I think you would be surprised to find how many people walk up to groups of horses in the open with treats or food and wonder why they almost get run over, kicked or do actually get hurt. Part of my job on a daily basis is walking the fields and checking everybody between feedings. You need to be very dilligent about being the boss when you're out with a group, don't allow any pecking order BS around you, especially if you're leading or bring in an underling. Think of any horse you're leading especially those low in the order as under your protection, if another horse makes a move towards it or pins ears/ bares teeth be sure to make it back off.
Good luck!