If you have the 6 ga, it wouldnt hurt. However, the size wire choice is influenced by the ring terminals that you can find for the alternator post. For instance, if the post is a small 1/4 thread, it may be tough to find a ring terminal that fits a big wire to little post. So, first figure out what size the alternator post is and then look for the correct sized ring lug...and buy that sized ring for the largest wire you can. For instance, if the alt post is 1/4", you will have some choices for wire that will probably be 10 ga and smaller. Usually 6 ga ring lugs take a 3/8 bolt.

Depending upon the make of car, there should be a wire from the alternator output to the battery + and another wire from the battery + to either the starter (GM) or solenoid (Ford).

I used welding cable on our 200+ amp alternator and it doesnt get hot unless we are running a really high amp draw (like a Warn winch).

Mike in Tucson