I also got into wakeboarding after snowboarding for most of my life.
If your step son is ready for ballast, I'd fill all three tanks. If others prefer riding the smaller wake, they can always get their sets in before you fill the ballast. A good rule of how to disperse weight in the boat is 60% in the rear, 40% up front. You'll also begin to notice that weight matters for side to side as well. If one side is heavier than the other, the wake will wash on the other side. The solution is to move weight, usually in the form of passengers on the boat, to move towards the side that is washing. Small adjustments can make a big difference, and these adjustments should be done when whoever is riding is in the middle of the wake. If he or she is riding in the flats on one side, that can make the boat lean a bit towards that side, causing the opposite side of the wake to be washy, and therefore lead to unnecessary adjustments.
As for fins, I'd leave them on. Although I'm sure your son has good edging coming from snowboarding, the fins shouldn't interfere with that. Sometimes they help if you land a little flat.