Before I answer your questions, I want to ensure there is a clear understanding of what we are talking about. What you will set up in FastMail is
Aliases for incoming emails, and
Identities for outgoing messages.
The
Aliases for incoming emails for your own domain determines the account that should receive those messages (that is, you can choose to have different accounts associated with each email address, as you probably expect). It is possible (probably recommended) to have a wildcard
Alias for your domain to receive messages to addresses not handled by specific address
Aliases. If you handle all messages for the domain from a single account, you will just need a single wildcard alias to ensure receipt of incoming messages.
- Your FastMail login does not change.
- All sending addresses for your domain (which I assume is what you mainly mean by Gmail aliases) need to be set up as Identities in FastMail. There is a limit on the number of Identities you can have in FastMail, but it is a very large number, very unlikely to be an issue in practice.
- Creating new Aliases (for your own domain and FastMail domains) is straightforward. You need to ensure appropriate Identities (for sending) are also created, though FastMail will often helpfully create one automatically.