PoolMom,
I've been running with some DE in my zeo-something filled sand filter for the better part of a year now with no ill effects. That's the good news. The bad news is that I see little if any positive effect from adding the DE. My water is very, very clear, but at night I still have visible particles in the light field, with or without DE in the filter.

I will say that I get a faster pressure rise in the filter when I have DE in it. It's obviously catching more stuff, but it doesn't seem to make a bit of difference in the visible water quality.