Two thoughts on this, both of which imply that epoxy is overkill for your installation. First, by using the Kerdi, you're shower is going to dry out much quicker than most, being that the water has nowhere to penetrate past the thinset. Secondly, if you're still worried about mold and mildew, might I recommend switching from Custom's Polyblend grout, to either Laticrete's or Mapei's grouts. Both have added antimicrobial agents to their grouts which almost completely eliminate the possibility of mold or mildew. Used in conjunction with the Kerdi, it wouldn't be a thought at all, especially if you have a good vent fan.

Thanks Bill! In Louisiana, mold and mildew can grow everywhere! I didn't realize there wasn't an antimicrobal in the Polyblend. I hope that I can make the trip in to check out the colors of the Laticrete or Mapei.