It has already been said, but for you and your horses' safety sake never ever tie hard and fast to a saddle horn or even worse to your leading hand on the horse being ponyed. I ponyed my mare's foals when they reached 4-5 months old while riding her and it went very well. A time or two a foal would try to set back, but by keeping my horse moving forward the foal learned that it was getting him no were to resist and since he/she was not tied fast to something stationary it didn't do him/her any harm. This helped so very much when teaching the foals to stand tied at a tie rail later on.
The POA mare I had I would pony with my mare when I would have a need to do it. Neither mare acted "marish" with one another. The only other horse I ponyed with my mare was the TWH I had. I learned from him to never have a longer than absolutely necessary lead to keep a ponyed horse's head at my knee and most favorably at the left knee. He had obviously never been ponyed before and was not actually certain of what was expected of him, but my level-headed mare handled herself very well in dealing with him. At least my 2 horses and I didn't encounter any serious problems, but I learned to try it at home first then go on a road ride with them.