I see the chaffee as an early game medium/light tank with a more powerful gun than the stuart, about as much speed as the m20 and with paper thin armor. It would have to be an early game kind of thing like being able to get it out with a lieutenant or a captain.
It could force an early panzerschrek/AT weapon purchase but it's thin armor would also make it severely vulnerable and therfore be both a gift and a liability.
You would not make very many of them and at the same time their agility could cause alot of trolling...it's price would be off-set of course.
it'd be like the IR halftrack in the sense that it will only be utilized under certain circumstances.
Have the M18 Hellcat act as a more mobile alternative to the Jackson... Consider that the Jackson is supposed to do a massive load of damage to vehicles yet it does NOT!!! The M36 is supposed to be a tank destroyer after-all!
The M18 is fast and it's gun is good. Again the main problem is it's armor but since the USF is so micro intentsive it will fit in quite nicely.
76mm Shermans could be a simple upgrade to improve it's effectiveness against tanks but it also has a
decent anti-infantry ability by default. HINT:HE and AP rounds...
jackson still needs a slight buff in the penetration department damage is good tho but having it paried with the chaffee would make them better as the chaffee can see farther than the jackson can (maybe an "aura" of extended firing range for the jacksons when they work alongside light tanks?)