But we know so little about those races.
I can't recall any details on the Double Helix except vague references in Claw Marks and the SO fiction.
The
Mopoks, from what I can tell in WC1, are a relatively primitive race, with no desire for galactic conquest.
The Firekka are also a peaceful race, and generally don't even want to leave their flock, let alone their home planet.
The Mantu are on the far side of the Kilrathi borders, from what little I know of them. They'd have to go through the Kilrathi before they even reach the UBW and Confed.
Don't think I've heard of the Milrathi before. Looks more like a misspelling of Kilrathi.
I thought the Pilgrims had all but disappeared, or at least gone to their supposed 'higher plane of existence'.
On the other hand, the Landreich might be a reasonable foe... but also an ally.
But even if it were reasonable to have all these races be made into a hostile force for UE2, I'm sure it would be far too complicated for the design team to come up with vids and voices for all of them.