Hiring a database Dev
First, if you haven't built anything don't use MySQL. If you're doing a new product -- go PostgreSQL. It's better and free.

If you insist on MySQL, at least go MariaDB. While both have their advantages MariaDB is a fork and that's where the community is at. MySQL is Oracle's newly purchased pretend-to-be-open-source-friendly PR campaign.

There is nothing wrong with cPanel for you. That said, database developers don't use cPanel. If you're going to hire someone and they can't administer their database without a 3rd party GUI tool -- RUN.


  Is giving them that link and the login credentials safe? 


It depends on safe. When you give login credentials they have at least the level of access granted by intent so look that up. And if your software is out of date and they're malicious usually more than that.


  By safe I mean my website and files and stuff, they wont have any access to any of that or cPanel right?


At that point it's a cPanel question. Often times db-clients leak privileges because they're improperly sandboxed. I can't tell you how many times I get to a psql box which launches right into psql, and I can get out simply doing \!.