Ignore Diamond...I'm 100% certain he is someone's pet parrot that has somehow learned how to type, and he just repeats the same misinformation over and over again.
Soy milk is fine, but make sure it's real soy milk, and not that processed crap that's got flavorings, emulsifiers, etc... added to it. Sweetened seems a particularly bad idea IMHO. If you want to test whether or not dairy is affecting your complexion, why not just drop milk for a while (soy, cow, or otherwise)? You can always add protein into your diet in other ways.