Fuses have little to do with power output. Size based on the weakest wire or device in the circuit.
Your 100 amp alternator is chicken feed compared to what your car battery. That's the number one item that you want to use a fusible link downstream of.
You may want to redesign the system the system with all those electric loads your planning. Yes, an externally shunted ammeter is good way to monitor flow without routing a main power feed to the instrument area.
What is not clear is what you want the ammeter to show.
Automotive ammeters usually are on the battery circuit - they show whether power is going in or out of the battery, and how much.
It sounds like you're interested in power out of the alternator. If so, then you need a ammeter that reads 0 to 100, one direction only.