Not sure what hard fouling you're referring to reading about. Certainly some skepticism is in order when reading most any internet posts, but it also helps to pay close attention to the circumstances described in many of the posts. The stories you've been reading may actually be quite accurate but incomplete as to circumstances, or you may not have recognized the role those circumstances play.
The 'hard fouling' problem that's most common occurs when low distillate petroleum products (for example, gun oil) are left in the combustion chamber area when the gun is fired. (Key words here are 'low distillate petroleum products' and 'combustion chamber area'.) The temperatures reached in black powder combustion are not hot enough to fully burn the low distillate petroleum products; the incomplete combustion results in that famous substance tar, which can be a real pain to remove.
Having this problem in a rifle like yours can be quite serious because it's very difficult to see or feel the tar buildup in the combustion chamber area until it gets fairly thick; at that point you're in for some serious cleaning duty.
Another point to be aware of is the 'low distillate' adjective; the more refined, or high distillate, oils are either fully consumed in the black powder combustion or ejected without becoming tar. In general, mineral oil based cleaners and lubricants fall in this category.
Finally, the tar problem can be avoided even if gun oil is used by swabbing with bore with an oil solvent such as alcohol (Windex will also work) and then drying it just before shooting.
As for rust, well, it's a whole lot simpler. Clean the gun after using (there are a thousand different ways) and apply a rust preventative = no rust. End of story.