when i add new fleet to the fleet cluster, should i do again prepare db command ?

hi there, you don't have to

so just to be sure only when i add a worker to fleet, this is configured to the mysql with the fleet serve command?

you would have to make sure to point that fleet to the same mysql, if you have 1 fleet running properly, you can copy what you're doing there in terms of variables/flags passed to fleet serve to another worker