Thanks for writing in!
If I set my site layout to be full-width in the Customize section, will I still be able to add sidebars to select pages?
Yes, by going under your page editor screen, right under the page options you can select choosing a sidebar for that specific page.
I’m mainly concerned with my BuddyPress pages (I’d prefer them to be full-width) and I don’t see an option to set those to have no sidebar
Please go to your BuddyPress pages Edit Page -> Page options and you can manually set no-sidebar layout on every BuddyPress page.