M365 mail flow setup and user configuration.

Forgive me as I am new to this......

I have done a little searching on the forum and read the documentation but have not seen answers to my questions.

In setup, it says you can can sync your AD users, add users manually or import using csv.

Nowhere does it mention that you can do all 3 at any time.  If you select AD sync, are you stuck using only that going forward?

In our environment, we have users that are in our AD and have m365 accounts and then we have users that are not in our AD but have m365 accounts.

If you setup the AD sync, can you still add non AD users manually and/or using a CSV?

Other setup questions:

What if you go over the license count for email?  Does it stop working on all users or just the person that were added over the # of licenses?

Also, we already sync'd our AD for the endpoint, but mail flow setup seems to want to setup another AD sync again.  Doesn't mail flow just use the current ad sync already in place?

Thanks in advance to any help given....

-b

-b