Welcome aboard.

It is indeed a garter. By the look of it, it's probably a red-sided garter (Thamnophis sirtalis parietalis), or an eastern garter (Thamnophis sirtalis sirtalis).

How much of the enclosure does the heat pad cover? It should only cover about a third of the surface.

You can start offering scented mice at any time.

Is the snake handled often?

Stick with what it will eat, but keep offering it fish and mice. As much as it'll eat once or twice a week, but be prepared to limit the intake if it start looking like it may be overfeeding.