I'm in a very similar situation. We just made the decision to lease my daughter's lesson horse for 4-H next year. He is a 19 year old, 1000 lb labrador and since he's a beginner to intermediate level horse, he's in a snaffle.
I need to get him a new bit and the leader suggested a Tom Thumb bit, which my research led me to flinch...is it? I have an email in to the trainer/instructor/owner, but she's out on a trail ride and I starts a camp tomorrow so I may not hear from her for a few days.
This is a softy and I'm putting an 8 year old (while reasonably experiences, still new to this bit) on him with a new bit so I want to go as easy as I can with him, and frankly, I have no exprience with bits.