the MySQL hostname should be unique (and I’m pretty sure that we won’t allow you to add a hosted domain that has the same name as a database hostname, and vice-versa).
since adding the MySQL hostname adds an A record pointing to the database machine for foo.example.com, you’ll get something like:
foo IN A 220.127.116.11 ; database machine
foo IN A 18.104.22.168 ; web machine
this means that people trying to view your site will get very erratic results. (this has happened to a couple of customers in the past when bugs in our system have allowed them to add a subdomain with the same name as a database hostname).
if you’re feeling uninspired, just call it ‘mysql.example.com’ or something boring like that.