Is the Cosmic Multiattack possessed by the original Galactus a combat ability, or is it its own animal? If it is a combat ability, is it a special combat ability unique to that Galactus and The Spectre or is it a standard combat ability used in a modified way?

I ask because we're going to have a 3,000-point game at a convention this weekend. I'm going to run the Elders and I may see original Galactus, so I'd like to know if I can copy Cosmic Multiattack with In-Betweener's Polar Opposite Attack.

It is not listed in the Combat Abilities in the Player's Guide and it does not say he gets to use Multi-Attack Ability. So I would say it is like a Trait and you would not be able to copy it.

It isn't defined as a combat ability, so I'd say no.

Also, Spectre doesn't have it.

does the old G have it still or is it ruled to be in the new g

2004 Galactus is, as far as I know, the only character to ever have Cosmic Multi-Attack. And since it is part of his specific rules it has not been replaced by something else: he still has the original CMA.
Keep in mind, though, that it does not allow the free actions to trigger the same effect more than once.

exuse me sir j-just one more question. Then why does the old IG sentinel has to use the DOF sentinel rules then? If the original Galactus useing his old rules that come with him why can't he use the old sentinel's rules

Because the Player's Guide tell us. To be honest those that wrote out the Advanced Sentinel rules did so with the intent of using them with the Original Sentinels for a reason.

But on the bright side working together is a combat ability so cosmic multi attack by any other name would still give inbetweener 3 actions

At 3000 i would expect all the kauju moleman and both draculas