Ambient lights flatten out the lighting, it was used in very low intensities as a fill light just so you won't have shadows blotting out areas. FG and GI were developed for a reason, they are not a cure all for every lighting situation but this is an interior shot so I can't see why not. Since you say your scene is so heavy, I say get some stand in geo and keep your MR settings very very low when you're testing and when you're done, render from the command line.