My Fox Trotter mare fussed and worried with any bit I put on her with loose, moving parts. She also hates tongue pressure.
She does well in a mild curb bit with short shanks and a port. I lucked onto the Myler #33 mouthpiece (HBT shanks) and she trail rides great in it. My Mustang also likes this bit so now it is my favorite bit. But I have also ridden my mare in a mechanical hackamore and she did okay in it. Not as good as the Myler #33 HBT shanked bit, but she certainly rides fine in hackamore.
Actually, her son, a 3 yr old gelding, is still very green and I ride him in a mechanical hackamore all the time. He is loosing his baby teeth and fusses with a bit. He will go good in a bit for a few months, gets fussy with it again (due to his teeth) and then I will switch to a hackamore for a while. I really can't comment how he gaits in it because mostly we are walking and I am just working on him being a broke trail horse, but he does trot out fine in it. We really haven't worked on finesse gait work yet.