I have a noob CSS question
 

Can I have my wordpress sidebar use a different CSS style file than the rest of my site? If so, how?

You don't need a seperate file... but if you wanted one, you would just need to use your stylesheet call again, and make sure that your sidebar tags are using exclusive names not used in the original WP style sheet.

My advice... just add the styles you want into your existing style sheet... save yourself some editing headaches down the road.