About the only sure way to tell is to give it a try, but it should not be a problem with ignition.
I don't like anything larger than 3Fg for priming my flintlock rifle, but for my muskets and trade guns there doesn't seem to be a problem. I have noticed, once in a while when trying out 2Fg in my rifle, that there was a delay. I think it was from a worn flint edge tossing few sparks, and the spark bounced a bit before hitting the powder. So, really, it wasn't a good test and was more a flint problem than a powder problem. I didn't like it at the time and went back to 3Fg for prime and main charge, and no worries, but that doesn't mean my analysis was correct.
LD