Kravak Scenario idea

I'm trying to develop a relatively simple scenario involving the Kravak.
Here's the general idea.  NAC and ESU are fighting each other in a 
standard meeting engagement between equal forces.  After a few turns,
the referee (me) introduces some Kravak ships into the battle.	The
Kravak's goal is to destroy any and all human ships, both NAC and ESU.
The NAC and ESU players won't know about the Kravak from their initial
scenario briefing.  All they know is that victory depends on destroying
the other player's fleet.  The Kravak will be a complete surprise, and
the victorcy conditions will change when the Kravak arrive.

A websearch turned up Indy's page which included a scenario entitled,
"Phalons off the Starboard Bow" which is similar to what I have just
described.  However, Indy's scenario doesn't list any Victory
Conditions,
and it is not clear how powerful the Phalons are in this scenario.

So, I'm hoping this list can help me develop this scenario by answering
a few questions.

1. I would like the NAC and ESU players to get the feeling that it would
benefit both of them to put their differences aside and attack the
Kravak
together.  However, the NAC and ESU players can still score points by
destroying each other's ships.	It is a short term gain that might hurt
them in the long run.  How would the Victory Conditions support this
idea?
What if I gave the players double points for destroying Kravak ships?

2. How powerful should the Kravak fleet be?  Should it be equal to the
NAC
or ESU fleet?  Or should the Kravak fleet be equal to the NAC and ESU
fleets
combined?

Thanks for your time.

--Greg
gregwong@best.com