Here's the problem I have with tailored lists.
A) If you and a friend are playing a game and you both tell each other what you are bringing, army wise and list composition, someones going to get the shaft and be completely countered with their list when the other player takes exactly what he needs to counter it.
B) Tau have "Good" anti flyer capabilities. If they take them.
C) I've seen Tau players in tournaments NOT take anti air and get completely stomped BY aerial units.
Here's my suggestion. If you haven't told him you play Chaos, don't tell him if he asks what you are bringing and force his hand at a TACs list. That way he could take a few Anti Air units but the majority probably won't be bringing it. Heldrakesa are amazing at taking out Marker Light units and Fire Warriors hiding behind an
ADL or cowering in Ruins.
When I play games, I never tell my opponent what army I am going to play, and I ask him to never tell me. I simply ask the bare minimum in mutual ruling we would both like to play with. Escalation y/n?
FW y/n? and point value. That's all you really need to ask. And that's all they really need to know.
Playing against a Tau player and saying you play Chaos can mean either one of two things (As you've stated above) "Do you bring flyers and hope he doesn't take EVERYTHING he can with Skyfire?" The answer would probably still have been yes either way simply to get that 1 or two much needed Torrents vaporizing his Markerlights. But you'd be at a much higher disadvantage if he knew what you were bringing.
I had a bloke once in the old
GW location I played at always ask what army his opponent would like to play as. To the point he was taking things like Ork Kann Walls against Dark Eldar Poison spam. It's not fun to go up against. It's a massive waste of time.
Now I get that most people don't have the expendable cash flow to play more than 1 army, let alone have enough points to mix and match everything from their entire codex. I understand that. I've played games where I don't use any Heldrakes not knowing WHAT my opponent was bringing and wasted a great deal of their points for said Skyfire. I've also taken Heldrakes thinking my opponents will take normal Troop choices and they walked out with Terminators
lol The points is, the game is more fun when things are layed out as simply 2 armies fighting, which armies are coming is the mystery