You're Outta Here, Slowpoke. Any unit in your mobile Tau army that can't move at least 12" will be quickly left behind, isolated from the rest of your force, and on its way to extinction. That means no infantry or Ethereals without Devilfish transports, no Broadside Battlesuits, and no Sniper Drone Teams. Kroot can make up for their limited movement with Infiltration, but be careful. Your carnivorous allies have their uses, but they will need support from other elements of your mobile Tau force eventually. That effectively chains those elements to the Kroot position. You have to decide if that risk is worth it.

Obviously, there are some disadvantages to an army list like this one. Broadsides are perhaps the most-efficient anti-vehicle units in the Tau army list, while Sniper Drones have their uses, too. Plus, the cost of each Troops unit grows dramatically when you have to take a Devilfish for all. Lastly, don't forget what counts as a scoring unit for the mission objectives (your numerous transports don't count). Mobile Tau have fewer scoring units than a static Tau army does, so protect them well (or bulk up on Kroot).

Economy of Force. With a mobile Tau force, you should almost always be able to achieve an economy of force with every engagement on the battlefield. What does that mean? "Economy of force" means that you should strive to use the right unit for the right job. It's a waste, for example, to use Piranhas armed with Fusion Blasters against a unit of Imperial Guardsmen. The mobility inherent in a Tau army like this one should enable you to position each Tau unit so it is shooting at the appropriate type of enemy.

Overwhelming Firepower. As mentioned elsewhere in this tactics article, the best (and sometimes only) way for the Tau to win is with concentrated firepower. Since most enemies can pose a close-combat threat to the Tau at even 25% strength (they'll lock you up in close combat at the very least), the Tau should always try to obliterate a target unit rather than just wound it.

The other reason you want to blast 'em to oblivion is the Victory Points Table in the Warhammer 40,000 rulebook. Though this table isn't in effect for all missions, you'll still encounter it enough to make it a consideration. Any kills that reduce the enemy models by more than 50% are useless unless you wipe out the unit completely (reducing a unit to 1% of original strength gets you only half points value, while 0% gets you the whole points value).

On the other hand, be careful of overkill here. Certain low-Leadership armies can be driven off when they start taking shooting casualties, so you don't always need to concentrate your fire to destroy them completely. Still, its better to stray on the side of too many shots than too few with a Tau army.

Your mobility allows you to bring the fire of several units to bear on your target regardless of your original deployment (for the most part). A static Tau army can usually only concentrate the fire of units that were deployed next to each other. That is perhaps the greatest advantage of the mobile style over the static style, though it also makes your life as commander more complicated. If you can master your timing so multiple mobile units can strike simultaneously, victory will be yours.