You have an existing account (A), which doesn't need to receive any granted levels.
And four new accounts, B, C, D, and E.

I'd probably try to concentrate levels on B then.

Link like this:
B -- C -- E
B -- D

I'd still link from A to B.
It doesn't hurt.
When B pays for the first month of the subscription, A is credited for a month.
And when B pays for the second month, A gets a free epic ground mount - Zhevra.

Log into A's account management.
Send an RAF invite to yourself.
Use the code from the email to create B.
Then upgrade B with a retail copy of wow.
Then from B's account management.
Send yourself two RAF invites.
Use these two codes to make C and D.
Then upgrade these with retail copies of wow.
Then from either C or D's account management, send an RAF invite to yourself.
Use the code to create E.
And then upgrade it with a retail copy of wow.

Because there are two links in your chain...
B -- C -- E.
You'd ideally want to boost a multiple of four toons.
Boost 4, Boost 8 etc.
For the granted levels to cascade into full toons.

If you boost 4 toons with A.
You end up with:
B x4 (Boosted) + x2 (Granted from C) + x2 (Granted from D) + x1 (Granted from C, based on the levels which were granted from E).
C x4 (Boosted) + x2 (Granted from E)
D x4 (Boosted)
E x4 (Boosted)

Totals: B x9, C x6, D x4, E x4 = 23 toons, from boosting four sets of four.



It doesn't really matter which toons are receiving the boosting.
The DK will plow through everything to at least the mid 60's instances.

Certain combinations are better then others, to work as a team.

4x Shaman or 4x Druid, will run quite well with a DK tank, for PvE.
In the shammies case, I'd run 4x Elemental.
In the druids cast, probably 1x Resto and 3x Feral or Balance take your pick.
4x Ret Paladins along with your existing DK is a strong PvE/PvP team.

If you specifically want 4x Druids, 4x Shaman and 4x two other comps.
I'd go 4x Ret for one of the two.
And probably a mixed team for the others.
Although 4x Disc Priest, 4x Hunters and 4x Warlocks would all be quite fun (have all of these).