I finally started messing around with a Traditions .54 cal. inline I got a few years back and ran into a few issues. The local gun shop did not have a lot of stuff for the .54s, apparently .50 is a lot more common. I bought what they had, assured that it would be fine for my gun. I still want to be sure though and thought maybe somebody on the forum might know. My manual recommends No.11 primers, is there a difference between No. 11s and the 209s (Win.777)? Also my manual suggests pyrodex or ffg, but can I shoot the preformed pellets as well? And with regard to the pellets, for.54 they come in 60 gr. pellets. So I either shoot 60 gr. or 120. 60 is not enough, but is 120 too much?
You can get 54 cal sabots online directly from Traditions.
You'll get more consistent ignition of pellets from 209's, but your gun has to be set up for them. I think Traditions has conversion kits still available online. If it's set up for #11's that's what you'll have to use unless you change the breechplug to one set up for the 209 style of primers.
The 54 cal pellets are fine with #11's so long as they're "fresh." If you live in a wet climate, between uses I'd be sure to seal the open box in a ziploc with the air squeezed out and a dessicant pack if you can find one. If not, I'd instead pack a dozen or so at a time into vacuum seal bags. Lots of hassle, but necessary for storage longer than about a month. Less an issue with 209's, but still a concern. I'd certainly start each season with a new box of pellets. Traditions will undoubtedly have load recommendations for your gun, but I doubt 120 is too much.
I've moved away from pellets entirely with my 54 inline (Knight). I've got the parts to use either pellets or loose, and greatly prefer loose. For some reason storage of an opened can of loose in a wet climate isn't the issue that it is pellets, plus loose lets you use charges in between 60 and 120. My rifle does best with 100 grains of 3f behind 300 grain bullets, for example. It's also a lot cheaper to shoot. I just use those little quick load tubes to hold premeasured charges on hunts, and it's as fast or faster than pellets.
You DO have an unusual gun in 54 rather than 50, and that's good. I've got both, and the 54 definitely outperforms the 50 in most respects. If you leave the sabots behind you can get away from pistol bullets and into some real heavyweight bore-sized all-lead conicals such as the TC Maxi or Hornady Great Plains. I'm a whole lot happier with them than with pistol bullets for heavy game. You can't shoot them as fast, but for really long range shooting they're actually flatter shooting and hit with a whole lot more authority. Also cheaper if you get into casting your own. You might have to put a lubed felt wad between the powder and the bullet to get the best accuracy, but that's a small price to pay and minimal hassle.
If you want to get into some real fun and cut your practice costs with that 54, try some patched round balls. I recommend .530 Hornady balls and .018 patches. You can get startling accuracy with charges up to 60-80 grains of 3f and fire 100 shots for less money than you'll have in 15 or 20 sabots. They're terrific small game loads for head shooting and virtually recoiless for practice, too. Go over 80 grains and the group size starts to open up. Still plenty good for deer to 100 yards, but not as tight as a conical at that range. I've whacked plenty of deer with a round ball over 80 grains of 3f, and never had to shoot twice. If anything, those round balls kill even quicker than conicals inside 100 yards, no matter what the advertising says. I've recovered a very few from deer that expanded as big as a quarter and stuck under the hide on the far side. Talk about kills!
My only regret about my 54 is that it's not a 58. I've use that cal lots in traditional side lock muzzleloaders inside 100 yards, and it's a real eye opener on large game. The only thing better would be a 62, 69 or 72 caliber. I've got a 75 caliber smoothbore I'm planning to use on moose next year, and it's a sincere hoot to shoot. Range will be limited to around 50 yards, but as an old archer and handgunner, that's part of the fun.