I can't answer for soy products as I don't use them myself. I have seen recipes that suggest either/or for whey and soy, so my suggestion is to make a half-batch serving and see how it works.

The protein is only there for purposes of ... well ... protein! So it shouldn't affect the outcome, IMO.

If anyone else knows a definite answer here I'm interested to learn, and if you try it, Avelev, please note your findings.