In my opinion, and this might be against the common trend, but if you are writing something meant to educate others, then you should reference books and the primary literature.

But to offer an answer to your question, yes, I think that a caresheet can lump all garter snakes together, but should perhaps also offer species specific information as it applies. I would also advise your readers that caresheets are meant as a starting point for finding information on a species, and that if they plan on getting a pet, they should continue with further research.

Ian