No reason you have to use a traditional style sidelock smoke pole, but if you use a inline the cap has to be exposed to the elements when your cocked and locked. I choose 54 cuz thats what I like but your right a 50 is just fine and I believe the minimum caliber you can shoot for elk..... 45 is minimum for deer.
When I said traditional I ment that on muzzleloader only hunts you have to have open sights, scopes are prohibited, sabots are prohibited, your projectile must fit within ten thousandths of the barrel or something like that, most conicals are okay, and of course the old patched round ball is good to go.
I personally shoot the power belts awesome accuracy in my two rifles followed by the TC maxi ball, but they quit makin them in 54 cal. Pellets are prohibited it must be loose powder or black powder substitute. I like triple 7, but pyrodex, or any of the modern substitutes are okay as long as your loading loose powder.
209 primers are prohibited.
There are some awesome muzzleloader only hunts in Idaho if ya can draw em.