[Major Doom]

How would Maximum Armor Modifications (Arsenal, page 44) affect Form-Fitting Body Armor?  Is it handled separately from the additional worn armor or combined?

[Mäx]

Separately so that FFBA has it own set of modifications.
Combined wouldn't make any sense, as you can wear it under any number of different armor and clothes.

[Major Doom]

Okay thanks.   Would the modifications stack if Form-Fitting Body Armor is worn underneath worn armor, which itself has modifications of their own?  For example, Fire Resistance modification on both the Form-Fitting Body Armor and Armored Jacket?

[Bradd]

Where practical, I would apply the effects to armor separately. For example, if you have no element-resistant armor, you get half Impact armor. However, if some of it has resistance, that portion of the armor contributes its full value.

[Mäx]

Okay thanks.   Would the modifications stack if Form-Fitting Body Armor is worn underneath worn armor, which itself has modifications of their own?  For example, Fire Resistance modification on both the Form-Fitting Body Armor and Armored Jacket?

I'm pretty sure that by RAW those wont stack and you just use the higher value.
But as i'm an advocate of full armor stacking in general, i think that a house rule letting them stack would be a good idea.
I think its thematically much better if my character for example loses half of his fire resistance and half of nonconductivity when she takes of her armored long coat for the meet, instead of all fire resistance and none of the noncondactivity.