I have yet to try Vespid. I just can't see them getting across the board to be effective. A great advantage of Tau is the ability to deny your opponent targets. I am thinking a unit of Vespid with AP3 weaponry trying to cross the board for range, would make a large and lonely target.
Sniper drones are aces. I would run a huge unit of 9 every time if I did not depend on my 3 Hammerheads so much. They are the bane of expensive MEQ units. That said, with the plethora of 4+ cover saves these days, sometimes they leave me wondering.
20 man unit of Kroot. Yikes, your brave. I would probably run 2 10man units. That way if they break (ld7), you only lose half of them.