Special Ability balancing

I thought I'd take this opportunity to talk about balancing special abilities.

Turtle:
As it stands, the Turtle tree is fairly balanced. Well, the scout side of tree is a little more useful, as bases are less important for two of the teams, and the speed penalty on the Heavy aircraft is quite high.

The latest changes I have made for this makes the heavy side of the tree recharge its abilities 30 seconds faster, so it much more useful against fortified positions (by virtue of higher damage). Also, the heavy side gets damaging ability 1 rank ($4000) earlier.

Rush:
Rush abilities are a bit harder balance, since a lot of them are situational. When playing against Turtle, the AF upgrade for Rush Anti-Tank Infantry is still quite popular. Beyond that, the Light Tank Assault Gun seems like a fairly popular choice.

I tend to choose the medium tank sonic barrage also.

Unsure how to balance this, probably associate a money cost with the upgrades.

Stealth:
During the course of a match, stealth has access to every unit available. However, some units should probably not be available at the onset, which have a cost of 3 (which require 1 rank).