Yes Ben, that's correct.
And, to answer your 1st question, the 12 bore rifle is completely adequate for buffalo. In India in the 1870s, the 16 bore was the common gun. It's 66 cal, and the 12 is 72 caliber. The "secret" is to use hard balls.
I am not intimate with the Petersoli, so I can't recommend a load, but I have made quite a few single barreled British style stalking rifles over the years and have found that the 62 and 66 calibers kill all manor of American game very well. American Bison, brown bears and moos all fall dead with properly place hard balls.
I will be making a 12 bore Rigby style rifle in the near future for just such a purpose.

I use a charge of 165 gr of 2F black powder in the 12 bore, but some shoot even better with even heavier charges. Ball size and patch thickness will vary, depending on the barrel they go in. Some with shallow rifling and wide grooves (Forsyth style rifling) can get along with a ball only .005 under bore and a thing patch. Others with deeper rifling and/or narrower grooves need to go as much as .020" under bore size and use patches of .016" to .022" thick.

Good luck.

Steve