Hello !
You can mix F-series and E-series licenses.
The user will benefit from both licenses and will have the highest usage limit allowed by either license.
For example, if license F allocates 100GB of storage and license E allocates 1TB of storage, the user with both licenses will have 1TB of storage (i.e., the highest limit).
Technically, allocating two licenses is possible, but it is strongly discouraged. This will result in additional costs for you and greatly complicate license management (due to overlap). Instead, try using add-ons whenever possible.
Please let me know if you have any other question !