Yeah, it doesn't matter what the shot size is as long as the weight of the charge is correct.
BP not only offers 6-1/2 shot but 5-1/2. I got some copper-plated 5-1/2's from them a few years ago, meaning to try them on late-season pheasants and sharptails, but have never gotten around to it!
The high antimony 7's aren't exactly common either, the reason I was somewhat flabbergasted to see a 25-pound bag of 7's at a local store recently. 7's are a really good shot size for lighter hunting loads, since the gap between 6 and 7-1/2 shot is the largest among common American shot sizes. Our 6's nominally run 225 to the ounce, while 7-1/2's are 350/ounce, a big difference. 7's usually go 290-300 per ounce, so plug the gap.
American 7's, by the way, are basically the same size as British 6's, the shot size of choice for their driven pheasant shoots.
