I wouldn't bother picking up new shot just for a trial run. If you get really into it, sure, optimize. But there isn't enough difference for me between 6 & 7.5 to notice.I already have some unopened #6....you think it's worth buying a bag of 7 or 7.5? Or just make do with what I already have
That being said, I recall that when I hunted dove a lot with modern stuff, I went to an "extra full" (think turkey) choke to ensure it was either a solid hit or a clean miss. I only had a couple come in close enough to get badly mangled by too tight a pattern. So if you're using a black powder gun with a cylinder bore, keep the shots close enough to make solid hits and have enough shot in the pattern to get the density you need at your given range.