I recently did a 4 tier cake using DH boxed mix. Cakes were super moist and yummy. But my fondant weighed the cakes down and made the tiers bulge out. Is the general rule of thumb to use a pound cake recipe for stacked cakes covered in fondant? Or should I have rolled my fondant super thin?

I ha the same problem when i would use the box cakes. I started using pound cake and i don't have that issues anymore. Its still good cake but it holds up so much better, especially when you have to carve the cake.

Your cake is really cool!
Did you let it settle before putting on the fondant? I made my dam really thick, so thick it's almost like playdough.
after flling the cake, I like to put something on top of it (if i make an 8in cake i use that pan to sit on top with a can of veggies or something) For a couple of hours, this will let the filling ooze out if needed and prevent the bulge.

There is a really good doctored box mix recipe on here that i have used several times. It's the *original* WASC cake by kakeladi. It holds up well to carving and fondant. It's moist, dense and tastes really good and you can change the flavor many different ways. HTH!