I don't have any experience with those two, but I surmise the mushrooms will grow faster than the cabbage and try to smother it out if there is rock to grow on around the cabbage. Leathers are pretty tough though, so it might be able to hold its own. My advice would be to propagate the mushrooms.
I've got some hairy mushrooms that touch my Nephthea colonies. It generally causes some sort of irritation for the Nephthea (sclerites show, etc.), but has been an issue long term. As nanook mentioned, leathers are pretty resilient. I've got a pavona that like to constantly sting my Nephtheas in a different tank and the Nephthea doesn't show any real signs of concern.