a few questions from a newbie

[Ken]

melee/caster: caster had to stop moving while the melee needs to run to the target.

Make a spread-out button for the casters and press it shortly when they have to stop following. Press it longer if you really need to spread out.

[edit1]

melee/melee: positioning of my 2nd melee was troubling me.

Aside from the fact that melee is multiboxing-unfriendly, I solve it by:
- attacking a mob with main
- running backwards with my main until the mob is inbetween my followers and my targetted mob
This means that the chain "you -> mob <- followers" makes 90 to 180 degree corner.

Another question is, if you multibox with different classes how are you using your actionbars with them? In a Warrior/Shaman combi i cant see any way of using the same bar/hotkey with different skills.

Find out which skills of different classes you can combine and put underneath the same key. For example: priests and warlocks have different area-of-attack casts and the can be under the same keypress. The same goes for class-specific buffs(e.g. armor of warlocks and 20x damage reduction for priest).
Warriors are very different compared to shamans so I can imagine there isn't much overlap. In this case you will have to use more different keybinds and/or use key modifiers to put more actions underneath 1 key.

[edit2]

If you don't have enough space for keybinds, you could make macros that combine skills, so you could have a button that does a normal attack when pressed normally while pressing shift together with that button would do a charge-attack.
This is done with modifiers in macros.