A. Yes, you can mix and match different Office 365 plans. Please note that there are some license limitations at the plan level.
The Office 365 Business, Business Essentials, and Business Premium plan each have a limit of 300 users, whereas the Enterprise plans are for an unlimited number of users. For example, you can purchase 300 Business Premium seats, 300 Business Essentials seats, and 500 Enterprise E3 seats on a single tenant.