Hi Judge!
I certainly understand your inability to get better cuts than your grocery store offers. As an option, there are online sellers that will ship really good meat to you.
Your plan is good, except the foil. No reason for that in an SI...just make sure you have a water pan in place on the floor of the smoker, next to the smoke box. Now, keep in mind that small cuts really have a mind of their own! They can take much longer, per pound, than ones 3x as big! I once smoked a 4 lb corned beef brisket that took 12 hours! 3 hours per pound! That's when I learned about small cuts. This is pretty much limited to any type of cut that will go through a "stall" (where the temp stops climbing as the internal fat renders in the meat). So, pork butts and briskets that are really small will be unpredictable on time, so be prepared!
Smoke it at 225 until it hits 190. If it's a flat, it will definitely benefit from the brine! Since you are brining, don't let the rub sit on it overnight...it will be too salty. Rinse thoroughly out of the brine, pat dry, mustard and rub an into the smoker!