Speeding Up

The Battleforce also lacks a form of Fast Attack; meaning that you'll have to get some yourself if you want to use anything.

I've already toldyou how to convert spare Fire Warriors into Pathfinders, so that should be of little concern.

I do not recommend the use of Vespid, and Gun Drones (while severly underestimated) are not recommended either. The latter is usable, but their effectiveness is mostly as an annoyance unit, meaning that they are not that great to take until you've filled your other points with Scorers, Railguns, and Crisis Suits.

That leaves us the use of Piranhas.

Piranhas are useful in a wierd way to the Tau army as they don't bring a lot of firepower, and because of this their mobility seems rather useless; I can bring one Fusion Blaster and Targetting Array there for 70 points? And they seem rather fragile compared to most other vehicles. Weakest possible armor on the side and rear and overpriced compared to Landspeeders, as well as poorly armed in comparison.
So why use Piranhas? Because they make excellent roadblocks. Piranha roadblocks can be used in two ways; the Swordfish and the School of Piranhas.

The Swordfish tactic is simple; A piranha with a Fusion Blaster and Targetting Array rushes the enemy and parks directly in front of the enemy's vanguard vehicle, moving flat-out. The enemy can attempt to destroy it in his shooting phase, but the movement prior will be slowed by him circumventing your vehicle. If your vehicle lives, then you can hop to the enemy's rear armour and fire your meltagun, or plop down in front again.

The School of Piranhas uses two or more Piranhas (two are a cheap annoyance; three or more are a tank murdering unit that should be feared). Each Piranha is armed with a Fusion Blaster (you could use Burst Cannons, but I feel they are a waste of the Piranha), Targetting Array, Flechettes, and some should have Target Locks. The strategy begins the same as the Swordfish; rush a tank, block path, melta and then annoy. But this becomes much more effective; the space taken up by two Piranhas is enough to waste entire turns of movement, and three or more can easily block mulitple units. In addition, the Target Locks will allow you to attempt melta on multiple vehicles; an extremely useful ability. Finally, the Flechettes will allow you to block off infantry paths; putting them between you and an offending unit will force them to move around. Assaulting them will not help your opponent either, as the Flechettes will tear light infantry apart, while hurting even the heavier ones (two Flechettes in a squadron while statistically wound every model in the assaulting unit! That means only 5 Boyz from a full 30 man mob will live, or you can shred about three Marines from a full Tacsquad if they attempt to grenade your Piranhas to death). In addition, when moved at cruising or flatout speeds, the enemy needs 6s to even hit the vehicle, which is easily failed.

If you are having trouble stemming an enemy rush, try using Piranhas in your list; from popping mech to Flechetting hordes, these models can annoy or kill any would be threat.