Tom thumb is actually a very bad bit to use in barrel racing, since the shanks are straight with no curve and the sides aren't their own like a real barrel bit or reining bit. It's all leverage coming down on the horses poll when your trying to make them turn.
If you want something similar to what you have but better, something like this Loose Cheek Black Snaffle Bit - Statelinetack.com
Or Three Piece Loose Cheek Snaffle Bit - Statelinetack.com
Both those bits sides are able to move freely and there more curve to the shanks so less leverage.
As for the being hard mouthed part, that's a training issue. You have to work on that. Doesn't matter how big a bit is if the horse doesn't want to listen to it they won't, not without the right training.
And there are no bit regulations in barrel racing.