However, just because this one 30 second shot, is grainy, does that mean that every shot has to be just as grainy to match?
In that scene, pretty much so. It would be jarring to go from a grainy clip to a pristine HD clip then back again. Yes, it can be a "stylistic" choice but all the clips in that scene will have to be the same before it looks like a choice (as opposed to a mistake!).
As the previous poster said, experiment a bit. You can throw in a third element to distract the viewer from the mismatch through the use of color -- for example a blue tint applied to all the clips in the scene. Maybe higher contrast . . . it's all about fooling people!
Good luck.