Pellets might work, but I'd never try it. If it works at all, it'll be lousy for hang fires and poor ignition like blhof said. I routinely shoot real BP loads from my 14" 45-70 Contender barrel. It is one h*ll of a lot of fun! You will be instantly hooked on it! Make very dang sure that anyone near you (and yourself!) are wearing good hearing protection. Trust me. It is unbelievable how loud it will be. I typically have people stop and stare open mouthed at the public range when I touch mine off. Think civil war cannon noise, and you're pretty close...
You need loose powder. A BP substitute like Pyrodex, Triple 7, etc, will work fine, but just trust me on this, you're really missing out on part of the experience. Use real BP. Just once, and you'll be hooked. The BOOM is better, the smoke is better, and the FLAME ball is obvious.
As far as load development, you're trying to make it way harder than it is. Depending on the length of the bullet you're trying to seat (ie, how heavy the bullet), will depend on how much BP to put in the case. I've shot from 300 gr bullets, to 535 gr postell's from mine. The only difference is how much powder space you end up with due to the different bullet lengths. If we use a 400-405 gr bullet for an example, start with filling the case with about 60 grains of powder. Then measure the distance from the top surface of the powder in the case, to the case mouth. This will tell you how far the bullet must be seated to butt up against the powder charge. If the bullet is longer than the avail space by a tenth or so, seat it anyway and you're fine. If the bullet would leave "air space", then just add 2 or 5 more grains of BP to the case, and measure again. It's very simple stuff really. You can't pack enough BP into the case to get into any real trouble anyway. If you severely over-compress BP, it will fail to ignite reliably.
You don't need any crimp at all for the single shot bbl, but I use the crimp die to just take the bell out of the casemouth from seating the bullet. I use a magnum rifle primer for good ignition and have never had a problem. You could probably use standard rifle primers and be fine, especially when you're just messing around like this.
If you want a more detailed load description, either let me know here or PM me with what bullet you are using, and I probably have a recipe you can take off and run with.
Enjoy, you are really gonna love it...