I never tried Pyrodex in pellet form, only powdered, and it has been a long time. I never got reliable ignition in cap and ball rifles that have a 90 degree bend in their flash channel like my T/C Hawken. However, your idea of loose real black under a Pyrodex pellet for better ignition is intriguing. The exterior dimensions of T/C .45, .50 and .54 cal. barrels were the same so you certainly are not going to burst the barrel. 100 grains of FF under a T/C Maxi Ball was my hunting load. 110 grains blew the hammer back to half cock. That's what sets the powder/pressure limit. If it's a worry you could fire a few shots at waist level to see if the hammer stays down.

With round balls the other limiting factor is the 48" twist in your T/C. It's a compromise that allows firing both conicals and round balls. It is hard on cloth patches. I only got good round ball accuracy from my T/C Hawken with reduced loads.

Pyrodex is great in Ruger Old Army revolvers.

Edit to add:
The not good enough for hunting ignition I got with Pyrodex was using normal percussion caps, not modern style shotgun primers. I prefer real black powder so I did not hunt down a nipple for using shotgun primers. The biggest benefit of Pyrodex is you can fire more shots between barrel cleanings and that is more important with revolvers that single shot rifles.