To be noted,
I had decided to level my hunter, and got all the mail attack power heirlooms for him. got him to 71 and decided i infact wanted to level my mage.

Opened a GM ticket saying that i got the wrong ones etc, and asking if there was a way to change the heirlooms.
a few days later i had all the emblems and shard in my mailbox :-)
not sure it will work for you, but worth a try if you get it wrong somehow.

I would also suggest the melee leather for the main, and caster leather for the rest.
That, or (if you're like me and don't really care about armor) get CLOTH gear for all, and don't worry about grinding heirlooms ever again,
after all, i use them mostly for the xp+ and trinkets.