Your Nitro should be just fine. For your piece of mind, by all means, start low and watch for extraction or opening dificulties before increasing the charge. Reducing the charge could be necessary to get the barrels to regulate, anyway. I'd start at your 100gr. load, if I were you.

I don't like cream of wheat as I've had a lump of it cook onto the inside of the case neck, which must be cleaned off prior to cleaning the brass and re-loading it.

I am a bit leary of placing a card wad between the powder and the filler form the standpoint that that wad might be viewed as an obstruction. A disk of thin card, like tablet backing might be just fine - if needed - probably isn't. It is best to have Black Powder in hard contact with the bullet and would mean more hard wads between the two when using lighter charges.