You need to set up Interact With Target for melee toons.

Go to your Blizzard Mouse Options {ESC > Interface > Game {Not Addons} > Mouse} and enable Click-to-Move on your slaves {Enabling click-to-move on the master is not advisable...nothing is more annoying than trying to click a mob and running off into the distance}.

Now go to Key Bindings {ESC > Key Bindings} and look for the Targeting Functions section. Near the bottom of that section, you will find Interact With Target. Set this on your slave as well.

Your Slave Combat macros should look like -

/assist MAIN
/cast hurtystuff

When you enter combat, hit your combat macro, and then hit the button you set for interact with target. Your slave will assist you and run up to the target without you having to position him.

Melee attack and channeled spells break autofollow, that's just how it works. Your mage will break follow if she casts a spell like Arcane Missles. I'd suggest getting Jamba and using the autofollow after comabt feature.

Edit - No you don't need ISboxer. It is a standard ingame configuration.