Unless you want to set up your own testing laboratory, it's going to be difficult to get an authoritative answer to your question. I can guarantee you that neither Schluter (maker of Kerdi) nor Custom (maker of RedGard) has tested the two products together. And neither of them will honor their warranties since you are following the installation instructions for neither.
Kerdi is a great product. You get a shower that is just about 100% watertight all the way up to the shower head. A shower that will dry out quickly because the waterproof layer is just behind the tile. A shower that does not have a drain with pesky weepholes that will clog. A shower that is really first class.
RedGard is a good product and has its uses. But a Kerdi shower is not one of them. You don't need RedGard. You don't want RedGard. Take the RedGard back to Homer.
If you start to have doubts, just close your eyes and repeat to yourself, "I do believe in Kerdi. I do believe in Kerdi. I do believe in Kerdi." until you believe in Kerdi. Your faith will not be misplaced.