Think of your team as 1 toon - if you run tank and dps, then the tank just has extra high damage output. eg. spamming 2 activates your single target macros, 3 activates aoe macros. tank/heals - you have a tank that can heal itself. heals/dps could be different depending on how you set up your healing.
Especially when starting out, you don't need to have all abilities available on all toons. Keep it simple to start, and add abilities as you see the need.