i never heard of the bubbles killing soft tissue corals. i have witnessed salt creep though, and it's a P.I.T.A.
i would be concerned about a sponge over the exhaust port of the skimmer, or any pump outlet. if you choose that option, i would check it regularly. i imagine a clog would empty quite a bit of water out of the tank and on the floor.
any sponge material or filter socks i had on my systems would clog in just a couple days.
it will also effect the efficiency of the skimmer.