Ideally, you should have clean hair and scalp for the optimal absorbtion of the minoxidil. I use toppik and for my AM session, I simply apply minoxidil 1st thing when I wake up, then wait an hour or so for it to dry, then apply the toppik/hairspray. Obviously for my PM minoxidil session, I shampoo my hair beforehand so its not an issue either.
If you apply minoxidil to your scalp with toppik already on your hair every once a while, I don't think this is a big deal.