Since you basically have seven toons, but are considering five boxing.
Really take a hard look at Recruit a Friend.

Assuming you have one account now.
Or even two works.
Add the remaining two or three.
Either:

A > B > C > D > E

A > C
B > D > E

Have your choice of Mage, Pally (Prot) or DK on "A" to boost with.
And level a whole bunch of 4x combinations, to 60th via instance/quest boosting.