To each his own. I am not a fan of Pyrodex, and certainly not of using pistol powder in any rifle! As long as you dry the gun and remove all the oils, or grease from the breech end by flushing the barrel and breech with alcohol, I suppose you will have no problem igniting Pyrodex. Not knowing what kind of gun you are using, I won't comment further.
OP wads can be obtained from most of the suppliers. I bought mine direct from Walter's fiber wads, but Track of the Wolf sells them, and if you go to Track's on-line catalogue, you can see good pictures of the various thicknesses available.
The OP wads you describe that come in a package are FELT wads, intended to be used in revolvers, for the purpose you describe. Only testing in your rifle will determine if they will serve a purpose of sealing gases well in your gun. Again, a chronograph is very helpful when trying to adapt a pistol product to use in a rifle. You may have to use 2 of those felt wads to get a good seal in your rifle. The OP wads are cut .010" larger than the caliber of the wad. That allows the wads to get down into the grooves of your barrel effectively, so that they keep gases from breaking through the grooves and cutting your patch/ball.
One of our members, Roundball, admits to using Felt Wads in his T/C rifles, and he says he buys the next caliber size larger than his rifle to make sure the wad seals well. I think that is an excellent suggestion.
The felt wads are flexible that you can get them down the barrel, even if they are oversized that much. The wads will form a " cup" on top of the powder, but that extra material will be forced backwards when the load is fired, and the gases push the wads from behind. That forces the extra felt into the grooves, and provides a great seal.
If you can't find a hornet's nest, but have wasp nests in the eves of your garage, or shed, or house, knock those down and use them. Granted, they are not as big as a hornet's nest, but the material is regurgitated cellulose, just as the Hornet's nest is, and has that same silky quality, and high flashpoint. Crumble it up in your hands, and pour the stuff down the barrel. It will tamp down when you load the PRB down onto it. ( Think of loading a pipe with tobacco.)
And, if you have neither hornets or wasp nests, try using good old corn meal. I used it with success for years before I tried the OP wads. Its a bit more flammable, but it breaks up outside the muzzle and rains down as dust, which doesn't carry much fire to start the forest ablaze. I have also used Cream of Wheat, and even dried Oats(oatmeal) as a filler( don't like the smell of the oats, tho').
