Brassicas are known to cause gas, so start with small amounts until your rabbits develop the gut flora to handle it.
Whenever testing a new food, I always use a "test rabbit" that is destined to be culled anyway. Just because the plant is safe for one class of animal doesn't necessarily mean it will be safe for others. Generally if goats can eat it, I consider it likely that rabbits will be able to eat it as well.