Optimum number of birds to get along well in a flight

[Finch Friends]

Last year when I bought my original set of them, a friend who came
with me to the same birdmart bought 4 of them.

3 of hers seem to be getting along fine with each other, but they gang
up on one of them and peck that one mercilessly.

My friend wants me to come and take the pecked one today. I am willing
to do it, but I wanted to ask you is there any certain number of birds
(BWM) that works better than not to keep together or is it just watch
and see who (which birds) do and don't live well together?

Right now my 3 BWMs are in a flight with a young (March 2008) Society
Finch male and (an accidental) Hybrid SFxSpice that I got from a
friend to live with the Society. All five are getting along ok and
none seems to be trying to dominate the others.

The 3 male BWM's all do their song n dance but don't really squabble
among themselves or with the other 2 larger birds.

I am wondering what will happen I add my friend's single BWM to this
flight.

Or would you suggest I move my 3 BWM males and the new BWM to their
own cage - I have that ability, if you think it'd give the one new
inductee a better chance at surviving. My friend doesn't know if the
one she is asking me to take is male or female, but I am reasonably
sure I will know soon enough when I get the travel cage in the same
room as the flight with the others in it. Thanks for any insights
you are willing to share....