My older girls aren't fond of a snaffle either. Too much going on with one...bar, tongue pallet pressure all at once. Another reason I like a curb bit, that curb strap gives a pre-signal before any pressure is applied to the mouth.
Let us know how she goes in the hackamore. Can your mom find out what she was ridden in by her previous owner?
She cleans up very pretty! Only 3? be sure to take it easy on her. I know I still haven't started riding my young gaited girls hard and won't till this spring when they are 4. I don't know about all gaited breeds but most have joints that take a bit longer to mature than non-gaited horses.