I think someone is equally an asshat if they say this codex is not official I am not playing against you..
On the other hand, if you're going to try out a home-brewed codex, I think you have to be circumspect and consider your opponent's feelings. You can't just drop it on them, or force them to play you. You can't really blame them if they don't feel comfortable at first. The burden kind of lies on you to make them feel comfortable and convince them you wrote something that is balanced and fair along with fun.
A tournament setting is no place to really be winning over people (if you win people are goig to wonder if your rules are broken, neh?) Also your opponent has no chance to choose to play your codex or not. Essentially they are compelled to play your list regardless of how they feel.
Spev and the others are right when they say GW rules are easily acceptable. (Anyway, you can always aim your gripes and bichin' about the rules at the distant and much hated GW. It is much more delicate if the author is your game mate.

)
I think you should write the Grot codex and be ready to accept lots of input from your opponents. If you can do that I'd be very happy to play against your Grot codex or try out the Tau codex you have in mind.