Tobi's suggestion: make all extension icons flat/solid — like the brand logos already are — so extensions clearly differentiate from the built-in line-style apps in the rail. Below: the flat set (B), the previous line-style set (A), and both at true 16 px next to the native icons. Native icons untouched either way.
The call: B (flat) or A (line)? Everything else — footprint normalization, per-app glyphs — is identical between the two.