I don't think there's a serving chart anywhere for a Topsy but I would say you'd be fine just adding a few extra servings. This cake actually looks pretty tall (taller than the standard 4") so you might just be fine following a standard serving chart. I personally would do it fondant. I've tried two in buttercream and it was sort of a pain. You just have to go slow with the fondant at the bottom since it's curving in.
I gauge and price by standard sizes. So a 6-9-12" x 4 feeds 100 ppl. Then for baking I go a little taller.... so for the 6" layer I bake a 6x2" and a 5x2" and then once they are filled it's really a 5" tall cake. This makes up for the loss at the skinny base and whatever you have to carve out.