Harnesses and walking jackets are rarely available with a breakaway feature. I've wondered how safe that is. Is a cat entangled in a harness much less likely to strangle herself than if using a non-breakaway collar?
Is it asking for trouble to leave a cat in harness even for a short time unsupervised?
In the Jacket (unless they have a leash attached) they should be ok... Rather tight fit.. Harness on the other hand, I would be very concerned with.