Those are good general rules and guidelines. Thanks for sharing them.
I’d add my own house rule: If players are trying to metagame the mechanic to gain an advantage by stacking armour, then as GM I’d impose a “your hero looks like a dork” rule to cancel the advantage.
If you add a shield, you’re going to be easier to hit because you won’t be able to dodge, and so on.
In addition, I’m no expert on armour but it seems to me that most of it is designed as a system, and there have to be some penalties to mixing and matching. In your example of a rigid chest plate and a soft ballistic suit, that soft ballistic suit better be designed to work with that rigid chest plate, or that plate might tear it. That solidifies your argument that it should be spec’d as a new suit type.
“May the Judge not convict you for cowardice!”