I have been staying out of the rumours threads for a while now, mainly because a lot of the information I was getting was plain wrong, and also because I was conscious of dropping anybody in it, but I thought I would chip in on this.
The Tau models have apparently been sitting around for quite some time now, some of them were redone over 2 years ago so I am not sure why they have taken so long to be released, but I suspect it's because the models range fits into 6th better than 5th.
They were one of the first armies to get a flyer (outside of IG's Valk / Vendetta), even though it hasn't been released yet.
As for an allies supplement, I was told that next year 40K will have a release very similar in format to the Warhammer Fantasy Storm of Magic supplement. In other words a book, cards and kits that could be used in multiple armies, in the same vein as the monsters from WHFB.
I assumed it would be a big supplement for Psykers, but it could just as well be for allies, as that seems to be the best way for GW to produce new kits without having to invest in a full 'Dex for them.
Now I hate to put Summer into any rumours, but I believe Storm of Magic was released about that time, which would put Tau and allies around June or July ish? However rumours strongly point to Eldar being early in the year, so I'm not sure if we would get 2 Xenos Codexes back to back? Unless GW really are going to do a release every month.