They reset their levels anytime you change their class/job. Making them the same as your main class just means you can gear them easier as you can just throw your older equipment onto them. That and they'll level better as you'll be gaining levels while going through the msq and thus they won't hit a cap until you reach max level. While on the other hand a DoL will need you to have spent time leveling that out as you don't have as many ways to level that vs a DoW retainer. You can switch their class/jobs at any time as long as they're not out on a mission. Not sure if you can change them if you haven't completed the mission. You also have access to two free retainers so you could keep one as the gather and the other as a DoW.