If they're just bouncing off each other, it's the easiest gig. Guest is a better choice at this point but it isn't like Reiner would be the reason it doesn't work as a director, if he did. If it doesn't work it's down to trying to be too modern and "with it," or because they're just too old to succeed to the level expected of them (which I'm not worried about in the slightest and am being realistic about).
Shearer is the only one I have concerns about, cast-wise, and that is solely because he's looking and sounding his age vs the others.