Does a Full Finance license automatically cover all activity users across the F&O solution?
That depends.
The D365 Licensing Guide (LG) shows a list of Finance roles that include Operations Activity. The list is too long to post here, so please log into Partner Center and go to pg.18 to view the full list:
In the above case the fact that the user is doing some SCM activities which can be covered also by a activity operations license, require this user to have an attach SCM or an additional Operations activity license?
It depends.
The D365 Licensing Guide (LG) shows a list of SCM roles that include Operations Activity. The list is also quite long, so please log into Partner Center and begin on pg.27 to view the full list:
If you do not find your customers role in the full list of Ops Activity roles, the Operations – Activity user license provides limited access to the Commerce, Finance, Human Resources, Project Operations, and Supply Chain Management applications. Operations – Activity use rights include all Team Members use rights as well as some additional rights that are listed beginning on pg.34.
Microsoft provides a cost-effective way for a single Dynamics 365 user to obtain full user licensing for multiple products via the Attach license. Licenses for products that provide core business functionality qualify as base licenses. (See the table below.) Each has one or more additional applications that are frequently used by people in the same roles and that qualify as attach licenses for that user. (These are sometimes referred to as subsequent qualifying applications.) See pg.4 of the D365 LG for more information.
Hope this clarifies! 
If this reply answers your question, please Accept as the solution to help the other members find it more quickly. Otherwise, please let me know if you need further assistance.
Regards,
Microsoft CSP Licensing Concierge
Were you able to log in and view the full LG?
As a friendly reminder, you're posting on a partner community; therefore, as a partner, you should be able to log into Partner Center and obtain the D365 LG that I referenced in my previous reply to review the role(s) that you're looking for. If you're unable to log in, please submit a support ticket.
As mentioned, the full list of roles is listed in the D365 LG and is too large of a snapshot to post here.
If this reply answers your question, please Accept as the solution to help the other members find it more quickly. Otherwise, please let me know if you need further assistance on this topic.
Regards,
Microsoft CSP Licensing Concierge
The purpose of mentioning the full list of roles for each license is for you as the Partner to use it to determine if the accounting and maintenance roles are listed for each license.
I can only provide you with licensing guidance.
1. Finance only - the D365 LG does not show that maintenance is included, however if you see that the accounting and maintenance duties that will be carried out are covered with the Finance license, then please proceed by purchasing this license.
2. Finance only + SCM attach - If the roles for the Ops Activity license are not listed on Appendix F, that would leave me to reason that the Finance + Attach license would suit my customer's needs, but again, I am not able to advise you on which license is best for your customer.
3. Finance only + operations - activity - Since I do not know which specific accounting & maintenance roles your customer will carryout, I strongly suggest that you review the info on Appendix F in the D365 LG to determine if the Operations Activity license will suit your customer's needs.
Hope this helps in your decision
If this reply answers your question, please Accept as the solution to help the other members find it more quickly. Otherwise, please let me know if you need further assistance on this topic.
Regards,
Microsoft CSP Licensing Concierge