I have a small quarter horse mare that has a low palate (thick tongue) and thick gums . She does NOT like single joint snaffles . For example, she'll push on the bit and almost yanking the reins out of my hands , she gets her tongue over the bit also , and she just doesn't like tongue pressure.
At my friends house (before I got her as a Christmas gift) ,she rode her in a long shank curb bit once (only time she rode her ) and she did wonderful in it . And the previous owners (not friend) they used a Tom Thumb (ick!) .
She is a trail horse, maybe a Competitive trail horse ,and just going to be all round .
My current bit is too small ( 4 3/4'' full cheek snaffle single joint ). Lesson learned on that needs to be at least 1/4 ''-1/8'' showing on the sides.