[Fingolfin II]

I'm inclined to agree with mormegil here. While, as the phantom points out with that quote from UT, Smaug could be used by Sauron, the control Sauron would have over him IMHO would be very limited- like Shelob, as astutely pointed out by morm.

I suppose the same thing may be said of the Balrog of Khazad-Dum also; if Sauron tried to persuade Smaug or the Balrog to his service, they would probably start out by following his instructions, yet Smaug's desire for gold and riches and the Balrog's desire to....kill....would in the end overcome their 'servitude' to Sauron and they would serve only themselves, destroying for pleasure. Not that this is totally out of line with Sauron's desires- only they probably wouldn't completely obey all his desires.