I have a ton of TTs laying around in all sizes imaginable, most of the came with bridles I've bought at tack auctions, but none have seen a horse's mouth here. I can't stand them. Thankfully her mouth isn't too big, so a 5.5/5.75" shouldn't be too hard to find.
Is this Weaver a good bit? Shorter curved shanks, not sure if the mouthpiece is what I want.
I'm sure it's a fine bit, most of the Weaver stuff is good quality, but it's not something I would want to use just because I don't like the style of it. I despise fixed shanks and I'm not a fan of the super low port unless it seems the horse needs one like that.
The overall shape you'd want though, is short, swept back shanks, and independent movement on the sides, so you can correct occasionally and give a clear cue.
Whatever mouthpiece you feel would be best, that's up to you.
If the horse's training has progressed to the point where she can be ridden in a curb, why not?
I'm a huge advocate for the snaffle. I honestly believe that most folks (and their horses) would greatly benefit from learning to ride well in the snaffle. I believe that BECAUSE there are not many people to have the knowledge and ability to use the curb bit correctly. I use curbs on all my horses and I see no problem at all with anyone else using one who wants to....so long as they have the knowledge and ability to use it correctly. I believe the OP does.
I know enough about the OP to know that she's got a good head on her shoulders, especially regarding the riding/training of her horses. If she believes that the mare is ready to progress up in her training to a curb bit, then I trust that she's right.