Whoever told you this doesn't know s**t about flintlocks and you should promptly ignor anything else they tell you and stop asking them for advice on flinters.
Pyrodex does not work well at all in a flinter. I know this 100% because I tried it and it scared the s**t out of me the first time I did it.
You don't seem to be well versed in muzzle loaders. I'd highly suggest you find someone who is experienced with a muzzle loader and hopefully a flinter to take to the range for a couple of times. These guns tend to be finicky and there are loads of potential pitfalls and mistakes you can make if you don't know what you are doing. Reading a book about it is not good enough.
I know this cause I made loads of newby mistakes with my first muzzle loader. Luckily it was a very strong modern caplock and I didn't do it or myself any damage. Flinters are a whole nother level of difficulty above the caplock.
I have a flintlock rifle and have fired it many times but only with real goex black powder. However since it can be hard to obtain real black powder sometimes I was wondering if using pyrodex would be an option. Some say a duplex load of goex and pyrodex would be fine, I dunno ? Never tried it, thats why I ask . As of yet haven't had any problems yet using real black powder.
If one were determined to shoot Pyrodex in a flintlock. they would have to use a duplex load. If you normally used 50 gr, load, you would have to put in about 10 grains of black powder first and then 40 grains of Pyrodex. The black powder would ignite first and then it would ignite the Pyrodex.
Once again Im curious because black powder isnt always available and supply is short.........
Thanks