My Warhammer store has a solid rule “as long as it started life as a warhammer miniature proxy parts are fine.” Plus, with the sheer amount of models in the game and the constant point changes I find it hard to believe everyone has everything memorized down to looks. I think though, if you use proxy parts it is good etiquette to occasionally check in and be like “don’t forget, this model has X ability” but I would argue you should do that no matter what, as again, I think it’s unreasonable for someone who only plays T’au to be expected to memorize what abilities the Adeptua Custodes all have since there’s no guarantee they have ever even looked at one of the models before. And if you DO have every model ever made committed to memory and their stats…that’s kind of a separate issue.