I've had blastos grow back from near microscopic pieces of tissue down in the skeleton, so they should be quick to heal from fragging. I would try to avoid cutting polyps in half, but as tight as they can grow it can be unavoidable. The ones that are cut should still regrow if they heal without rotting.
Blastos are also heavier feeders than most people give them credit for. I spot feed mysis every other day and they definitely show their appreciation with more growth, more babies, and bigger polyps. Feeding the polyps that aren't cut will help the whole frag recover much faster as they seem to share nutrients between polyps IME.