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Like Ally said, all 3. Its not acceptable for her to be kicking. Put a red ribbon in her tail as a warning, but do not allow her to kick. If her ears go back and she starts swishing her tail, you must redirect her attention (kick her, use a crop, clucking noisees..whatever works). Its not ok for a rider to shrug her shoulders and say "She doesnt like other horses" (not saying you do this). Be consistent! Dont just do this once, but every single time she acts up while riding or in halter (pasture behavior is different) good luck!!

She should be controllable in every spot eventually. But if this is her first trail ride in while, put her where she is most comfortable which sounds like the back if she's kicking out at other horses. If she goes too fast try circling or seesawing back and forth on the trail. Once you have been out and about a while, practice the other positions in the group with her. Let your riding buddies know she may kick and ask them to practice passing you giving plenty of room at first and a verbal "Passing on your left". Point her hind quarters a bit away from the other horse and be sure to reassure her/pet her when she is getting passed so she begins to change her mind about the whole thing. She may have been kicked or bitten herself in the past and so doesn't like other horses too close. If she shows any sign of aggression, be sure and reprimand her in a meaningful way that let's her know you have control of her and the situation.

If she kicks she should be ridden on the outside (if you are riding side by side) or at the back.

If she walks to fast for the other horses your are riding with then she'll just have to learn to walk slower. Her walk may be equal to a horses walk though because horses don't walk as fast but they have a longer stride than a pony.

I agree with Ally, The only way she will learn and be able to be compatible is to slowly and constructively be taught how to act. If you let her get too used to always being the leader she will think that its ok to misbehave if she doesnt get her way. Plus for the safety of the other horses ans even yourself, its a good idea to get her used to all three positions even if you have a more experienced rider help you train her. I'm not a huge fan of them, but if used correctly, I believe a hackney might even be of use in training her. Good luck, be safe, and I wish you two the best!

Ride in the front if she is the fastest of the group.

Tie a red ribbon on her tail and warn others to keep their horses a safe distance away.

If you ride in the back, practice teaching her a "sloooow" command.

All three. Its good for horses to practice in all places. She should be brave in front, and the others should know better than to get too close. However, you also need to teach her to slow down and be in the back.

If she kicks then she needs to be in the back. So when shes in the back and starts walking fast just say "easy!" If she keeps walking fast lean back and pull back and try to make her collect and back up. Then proceed to walk and if she keeps doing it again repeat.

Back, you should be aware about the other horses.

Control your pony child.