I was thinking about screening (basketball terms), when it all fell in place with fire warriors and kroot. Let's say you have a squad of firewarriors supported by a kroot squad, and they're going to capture a central objective.
You put both squads onto the field - kroot infront of firewarriors. They start moving towards the central objective. In 2-4 turns they will (should) get there. It is not likely that they will even be shot at but if they are you have 2 scenarios. 1 - they shoot the kroot (you don't really care - you take cheap ones). Or 2 - they shoot the firewarriors who get a 4+ cover save, meaning that they will probably survive.
Right, here comes the complication - an enemy unit is in your way or has beaten you to the objective. This is where the screen comes in handy.
Both open fire on the enemy unit (always do this - but shoot with firewarriors first unless within objective grabbing distance/it's Turn 5). Oh, and don't open fire with the kroot before the fire warriors as if you see your firepower failing as it often does, you will charge the unit, letting the firewarriors run past. Hopefully, you will also have some support, so you can pummel more shots into that unit.
SO... Basically the kroot assault (take some hounds) and with a bit of luck kill some of the enemy so they are not within contesting range, while your firewarriors jump onto the objective. If this works, then Tau can actually capture objectives and don't have to use devilfish!
I will be trying these tactics out with my unpainted/craply painted Tau (hope you don't mind) so stay tuned for updates. Oh and I have thought something up to combat nasty deepstriking attacks on your firewarriors too...