The issue I'd have with 6's for grouse: Not necessarily a problem if you're not also shooting woodcock. Especially with standard small bore shot charges, 6's would produce a pattern that could well lack sufficient density for woodcock. I'm pretty sure most hunters would choose something between 6's and 10's for mixed bag grouse and woodcock hunting. Likewise pretty sure that either 7 1/2's or 8's would be the most popular choices.
That is a concern, but I don't pull the trigger on Woodcock enough to be concerned about it.
My chief complaint about bout 7 1/2s and 8s is that I've found to many of them in the breast. 6's pass right through, although I do remember a grouse I shot in an area when lead shot was a no no and the #6 bismuth pellet split and I found both halves in the breast.
