gmail supports having +stuff in your email address too. it's fairly easy to set up filters to put stuff into folders based on the email it got sent to.
You don't use a '+' at all in the fastmail email addresses you give out.
For example, say with gmail you have name+stuff@gmail.com, with fastmail you could use that if you wanted, but you can also use stuff@name.fastmail.com
If fastmail receives mail on that address, it converts it for you as if it had been sent to name+stuff@fastmail.com instead.
This happens entirely on the fly so you can make 'proper looking' emails without a '+'
You don't use a '+' at all in the fastmail email addresses you give out.
For example, say with gmail you have name+stuff@gmail.com, with fastmail you could use that if you wanted, but you can also use stuff@name.fastmail.com
If fastmail receives mail on that address, it converts it for you as if it had been sent to name+stuff@fastmail.com instead.
This happens entirely on the fly so you can make 'proper looking' emails without a '+'