I have a Pro Site with a few other guest Admins. Will they be able to configure Pro features?

To configure settings (e.g. through the Site Manager) that come from the Pro package, the Admin also need to be a Pro user. Obviously you, as the Master Administrator of the given Site, have access to all settings, as well as other Admins with Pro accounts. Admins with free accounts can configure all non-Pro settings, but does not have acccess to custom domain settings, favicon etc.