You have A and B now.
The new accounts will be C, D and E.
Which will be 5 in total.

You will want to cascade the granted levels.
Since A is doing the boosting, I'd link from B (your first example).

B - C - D - E

Create the four new toons on B-E, boost with A.
Repeat the process for as many teams as you think you might want to try down the road.

1-60 with RAF is fast, take advantage of that.
60-80 is the same speed at any point.

I would seriously look at multiple teams to 60th.
It is very fast now, and won't be down the road.
With a geared boosting toon, you can go 1st to 60th in 10-12 hours /played.
In 3 months of RAF, you could easily boost a team to 60th each week.
If you cannot stand playing <insert classes> don't boost them.

But consider a melee team... pallies, rogues, ferals, etc.
Consider a casting team or two... shammies, druids, warlocks, priests etc.
Consider a mixed team for instances possibly...

Well, play whatever is fun.