I would use a varying grade of aragonite. CC is abrasive. I little bit would be okay and probably enjoyed by the BS because it would help fill some of the voids between the aragonite and the larger shells. Whatever you do, do not use a single sized aragonite. I would keep my eye out for some empty clam shells too (which BSs love to incorporate into their plans).
In regards to the egg crate, that is also going to be abrasive. Many times they will dig down to the bottom (or in your case the egg crate). A determined BS will continue to try and dig through that without success.
One thought, do you really want to go mixing stuff from all your tanks into the 90? If you have had ponies die in the past, wouldn't it be better to give them a tank environment that didn't have leftover whatevers that might make them sick? Don't know
