Yes, you can mix and match Microsoft 365 plans. Please note that there are some license limitations at the plan level. The Microsoft 365 Business Basic, Apps for business, Business Standard, and Business Premium plans have a collective limit of 300 users, while the Enterprise plans are for an unlimited number of users. For example, you can purchase 200 Business Premium seats, 100 Business Standard seats, and 500 Enterprise E3 seats on a single tenant.