10 GA wire for wiring the Ammeter

Depends. If you are bypassing the ammeter, and wiring in a non--stock fashion, you only need (functionally) 1 source wire to supply the interior. if you haven't or won't be adding many high current accessories, like a great big stereo amp, one single no10 is fine.

BUT if you are still running this through the connectors on the bulkhead connector, you should make use of the original two sets of connectors simply to share the current. Those 1/4" wide spade/ flag connectors were NEVER made for very much amperage

Best example I've used is that in HVAC I used to work on electric furnaces (mostly heat pumps) and they use the same 1/4" wide type connectors. Those carry nominally 20-25A to a heating element each, and THEY DO FAIL.

This is in part why, (if you look it up) that Chrysler once had what became known as "fleet police taxi" wiring for the ammeter. This was a partial bulkhead bypass, not sure if it was factory or dealer. It is documented in some of the service manuals.

"In the old days" about 1972-3 I had a 440-6 RoadRunner. The bulkhead connector failed, and I just drilled out the two ammeter wires and ran 3X paralleled no 12 through each (It is what I had, no 12 silver plated aircraft wire)