Yep, in fact, back in the 1700s there was only cannon grade and musket/rifle grade powder...Many of the tales we hear about today were created in the 1940-60s when muzzleloading got popular...

FFF is perfectly fine to use in larger bores...

Ive read that when fffg is used you should use 10 gr less powder to equal to ffg

yes,like in my hawkins .50cal i like 80 grs of 2f for my powerbelts but would use like 70 grs or so of 3f if i wanted the same pressures.but 3f may shoot good at 80 grs.
highest i have seen in a hawkins .50 cal with 3f is around 90 grs.70/80 grs seems to be norm with 2f or 3f for best accurate shots.