I think you should go ahead and do it. I wouldn't be worried about "learning" something new or whatever people are saying. If you can't learn to kick your heel out to disengage yourself, then riding a bike is too difficult for you and you need to withdraw from the tri and return the bike before riding it for a refund. I have never fallen over on clipless pedals. Have I come close? Absolutely. Mostly towards the end of a ride when I am tired and not thinking straight.

The only thing I would worry about is the transition. You will need to practice that more than clipping in and out.