There is a ton feedback that I have to chew through, and to little time to do it I would really like to read and evaluate from the point where @redrum and @Black_Elk finished a few games. And I would like to study the statistics of those games as well as hold that up towards what the AI does and handles the map. But that can't be today as I have too little time.
But just my thoughts on the unit prices and unit functions. As I see it, the Mech-Inf is ofcours a unit with its own special function and (to tow inf and art), and so is the SP-Artillery and Anti-Tank. They are also meant to be medium weight units in between inf/art and tanks.
Special for the Mech-Inf is that this unit aims to be very an attractive and widespread purchase. Purchase numbers should rank after Infantry and artillery, and maybe double as purchased than SP-artillery and Anti-Tank. So purchase tendency could be like:
1 SP- artillery
1 Tank (Light/Med/Hvy)
The Mech-Inf should fill out the role of an expensive infantry, that is mostly a good purchase, unless you are stacking defence, you have to walk a very short distance, you have to sail on transport or you are low on iron and want to buy ships instead or something.
It seems to my that the Mech-Inf as of now lives up to this, but I will have to study the saves and the AI further.
When it comes to the special units, AS, colonial, commisars and kamikaze, they are just bonus units sometimes available. The player should go "yay! A new commisar!". Unlike the marines that are always available and unlockable heavy tanks and jets. As the availability is not unlimited, they don't need to be "balanced" or have a niche role. They are just nice to buy when available. I don't see a need to change them.