BD, whether your sxs can handle the load you're using depends on the gun. And whether it's the best choice depends on a bunch of factors, such as the kind of cover you hunt, how you like to hunt, and whether you hunt with a dog (and whether it's a pointer or a flusher, and how well it retrieves).
There shouldn't be any problem using the Golden Pheasants in a modern (post-WWII) sxs. Older gun, that would depend on the gun. For me, 5's are the largest shot size I've ever used on pheasants, and living in Iowa and hunting them a lot, I have the opportunity to shoot quite a few. This year, I'm past 60 wild roosters in the bag, all of those but about half a dozen using an Army & Navy 2 1/2" Brit 12, Kent Pure Gold 1 1/16 oz British 6's (they're about 6.6 in American shot size) in the R barrel, 1 1/8 oz American 6's in the left. I was just looking through my notes, and my dogs have only had about 3 difficult retrieves this season--due to the fact I didn't center the bird, not any deficiency in the loads I choose. I should add that I hunt over pointing dogs, and I take the vast majority of my birds inside of 35 yards. Probably a good 80% inside 30 yards. And I'll almost always pass up shots beyond 40 yards, unless the bird's a crosser--they're far easier to kill than the long shots where the roosters are showing you only their tails and backbones.
But I wouldn't worry too much about square load theory. Shoot some patterns, and if what you're shooting looks good on paper, it'll probably work for you in the field.
