Alternator install/Charge Wire Questions

Alexhetti said
When installing the new alternator, I've read that it's best to add a new charge wire with a fusible link running from ALT to the BAT terminal on the starter relay. What gauge wire/fuse link is going to be best for doing this?

The safety issue will be that if the battery shorts to ground, it will take twice as long for the fusible links to open.
see Fusible Links in Charging Systems with Ammeter
The advantage is that battery charging current no longer shares the wiring and connections through the firewall multi-connector.
How much current does the battery draw during recharging?
Well that varies. If the battery is fully charged - ZERO.
If the battery is not fully charged, then it depends on how low it is and the alternator's output voltage.
Illustrated here

And will the existing wire from ALT running to the amp gauge be okay to stay connected?

It's best to leave it connected. Otherwise the power needed to run the car has to go through a bunch of extra connections as well as through two fusible links.