Having now played a small number of games in OCTGN, I'm switching my allegiances. My favorite deck so far has been an Eldar/Dark Eldar deck. I need to toy around with Tau a bit more, I had some disappointing initial attempts with that faction. I still really love the aesthetics of the Tau, but I have to be honest with myself: I'm much more a control player than a combo player, so I will likely continue to gravitate toward Eldar and Dark Eldar.

Definitely Astra Millitarum as my main deck with Space Marines as allies. Although Necrons and Tyranids are very close seconds and thirds when they get released.

Tau followed by Tyranids. I'm not fussed about what is the best combo, I'll just be playing my favourites!

Will dabble in all factions occasionally though.

Chaos! Though fancy poncy Eldar seemed kinda annoying to my opponent when I used them (we had a few quick games with a solo core set shared between us).

Probably end up with Nurgle with Ork allies, cos they are both green!

Well. I'm a fluff sort of guy and I might go for a Space Wolves and Catachan Jungle fighters centric deck. They can also ally which is nice.

I have some Dark Eldar models and I've always loved them, so they'd be my main. But also Eldar and Tau.

Something struck me as odd with those alliances: the whole orks teaming up with imperial guard thing, (yes its blood axe mercenaries but still. Why wouldn't the tau team up with ork mercenaries?) and the fact that space marines cant ally with Eldar, something wich they have done severall times in the background.

Why couldn't they have made it: Space marines-eldar-dark eldar-chaos-orks-tau-astramilitarum?

Something struck me as odd with those alliances: the whole orks teaming up with imperial guard thing, (yes its blood axe mercenaries but still. Why wouldn't the tau team up with ork mercenaries?) and the fact that space marines cant ally with Eldar, something wich they have done severall times in the background.

Why couldn't they have made it: Space marines-eldar-dark eldar-chaos-orks-tau-astramilitarum?

The Tau don't actually do mercenaries. The Kroot are mercenaries and Tau allies, but those are separate facets of their society, and one that is hidden from the Tau. Generally speaking, races that work with the Tau are incorporated into their Empire wholesale, not hired temporarily. Add in the degree to which Kroot, one of the Tau's oldest client races, hate Orks (it's a lot, by the way), and an Ork-Tau alliance is completely off the table.The Guard alliance makes sense because not only do the Blood Axes have an established history of mercenary work, but the Guard and Orks are both blunt instruments.

I personally have some misgivings about the Tau's place on the faction wheel, because as you say, Eldar-Space Marine (or other Imperial force) team-ups are a known thing in the fluff, and part of some of the cooler bits of 40k lore. However, I don't actually think there's a better place to put them. The Tau are in the best possible position on the wheel they could be with respect to the dispositions between the factions, even if that position causes some problems of its own.