Pretty sure ANY bit is allowed. A slow twist snaffle is definitely ok! Ans Really not that harsh of a bit. A double twisted is allowed too but I would never use one on my horse!!
My horse goes in a happy mouth eggbutt snaffle for all 3 phases.
Im yet to see anyone use those bits in any form of riding I know htey are not allowed here for eventing so look uo your local rules to see as if its in pony club or riding club there are bit restrictions.
I do 3disciplines in a rubber french link
With my ex eventer ozzie I used a slotted cheek joint kimberwick and a cavesson noseband.
With marco he is going to be in a Jp korsteel full cheek snaffle with a cavesson. If he's strong then something else LOL haven't taken him out yet so he should be ok in that. He listens to me and pulls up easy :)