Boosting with unlinked is fine.

Downside is that:

A - B
C - D

unlinke E

A and C wont be able to grant raf levels (and b/d dont get zebra), but you don't sound like you were planning to grant anyways. Option 2 is cheaper but granting is crimped.

Search for Deathwalker and Fastest (this site), you will see the fastest way to level up 5 new with your booster but its not good for the existing toons and will cost the most.