I don't think Gandalf knows that there is a Balrog in Moria. I think he knows there is something very powerful and evil, but probably isn't sure exactly what it is.
I feel that the Orcs in Moria are there just because it's a position of strength for them to base their raids from. Also I'm sure they see the Balrog as something of a god, so if they are aware of it, which I would assume they are, they are probably there to worship it. If they are not aware if its presence, then at least it's aura of evil probably drew the Orcs to Moria. I don't think the Balrog sees the Orcs as anything more than cannon (arrow?) fodder and is probably pretty disinterested in them, although it may make use of them when it sees fit.
I think that the Balrog is aware of the fellowship early on, probably even before Pippins little incident with the well, and it is setting up a trap for them as they approach the bridge. Things seem a little too well planned (the fire, etc) for it to just be a spur of the moment kind of attack.
But then again, most of that is just speculation and opinion on my part.
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