You should read the book again. The Orks do not run away when the Balrog does approach the hall. They go aside to let him pass. They do approach after him and the work of the trolls to get the orcs across the fire was continued during the approach of the Balrog.
That does mean the Balrog did at least cooperat with the orcs. And the Appendix said that the orcs were send to Moria by Sauron. So at least the Balrog and Sauron did tolerat each other and for me it is likly that they cooperated.
Respectfully
Findegil
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