I think like a lot of the people here that Tau will be ok for the moment.
Basically they aren't big enough to offer a huge problem for anyone if they did turn their force on them, but currently there isn't enough of a reason to do it.
The Imperium are far to slow to act, as ordered get passed down a long chain leading back to Terra, and by the time they would reach the front line the orders are of no use as they are out of date or are twisted by all the local bureaucrats to fit their own needs and funds / forces will have been syphoned off to their own pet projects. The ones at the end that are asked to fight the Tau will stall and delay as much as they can to save their own skin as they don't want to be in the firing line. Also things like the Space Marines and Mechanicum are shit scared on even looking at the tech of other races to think that they should conquer them and enslave them to make new stuff for them.
Chaos on the other hand could and probably would go in and steal their battle suit tech to harvest for themselves. Maybe someone like Iron Warriors would love to make new Daemon Battlesuits.
Orks are random, if it happens it happens. But the likely chance is, unless they hit the home world then some Tau will find a way to hide off somewhere till it is gone. Also that greater good thing will likely mean if the Ork force is big enough that they will sacrifice whole planets and systems to limit the damage and repel the Orks.
Necrons are a bit random as well. Not sure of the reaction between them, but in either case they seem to busy piecing their Empires together again, and unless that happens to be the Tau homeworlds as part of one of those Empires they won't go after them.
Tyranids would be a random act of genocide on their part. If it turns out they are in the way then they might be crushed, if it brushes by the side then they will be happy. But as Tau have little to no warp presence, it isn't as if the Tyranids will have a lock-on to them.