We have two windows server 08 machines, one of which is an AD slave to the other. Do the roles need to be mirrored if I was to say, add a printer role to one, or can they be kept fairly seperate in that respect?
I have no idea what you mean by "an AD slave to the other". If the two machines are both domain controllers (which is what I hope you mean), then no. They do not need to have the same roles, unless you're doing something with clustering or load-balancing. Which you probably shouldn't be doing with domain controllers.
Advice, not criticism - it sounds like you're admin-ing windows servers without knowing some really basic things about Windows and the ecosystem. If you're in the process of teaching yourself, then congrats, you're going to have a lot of fun. You should probably get the MS Press study guides for the MCITP Server Admin and Enterprise Admin. You don't even have to plan to take the test, but that's going to be a really cost-effective way to teach yourself about Server and AD, without making too many mistakes on actual servers.