I would say they are 4", 7", 10", 13" tiers.
That's what size my cakes are here (although they're all twisted to each other....
This cake has only 2" difference between the tiers.... (they're 4", 6", 8", 10", 12")
I'm afraid I have no experience of working in buttercream, so can't help you there.
I'm pretty sure you could put fondant swags onto a buttercream cake, so long as the fondant was thin enough to not be too heavy.
I think Relznik has the sizes. I have done a fondant swag on buttercream and it worked fine. (It's the silver cake with roses in my albums) You do have to decide where you want it and leave it there, even if it's on a crusted buttercream. To make sure it won't fall off you can brush some water on the back.