I'd like to recommend that in the next release their cost be changed to half that of the equivalent citizen soldier, plus 20 metal. So a mercenary archer would be 25 food, 25 wood, and 20 metal.
This video es from an older version but will help you alot with the basics. Might not get much what is he doing but If you watch it carefuly more than one time and listen to what he says, you'll learn a lot