You can show in romals unles the event rulebook states otherwise. However, I would think you'd want to give the judge less reason to think you're different. The more they are looking at what type of reins you have the less they will be looking at your horse. Although it is legal, that is just something to bear in mind when choosing your reins. I can't imagine it would be a huge deal with a good judge but you never know nowadays.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.