Hi Sam, This care sheet is a group effort by the members here, and really the best one for you to study:

Garter Snake Forum - Garter Caresheet

And yes, many of us keep more than one garter snake in a cage at a time, Just make sure they are either the same sex, or the same sub-species.

As for food, acceptable items are in our care sheet, I feed pinkie mice and trout, others feed fish and night crawlers, opinions vary, I'm sure everyone will be happy to give their viewpoints along the way.

Enjoy your time on the forum.