You do NOT need to put cardboard between the layers. Small is OK, though you want the bottom one the largest and the ones above it the same size or smaller. You will actually have more problems if you make big cakes vs small cakes.
Make sure you cut the tops off the bottom 2 layers before stacking.
Frost between layers - personally I would frost each shape a different color - but that's me.
You can just use a box mix if you want, it's not a problem.
I agree but to make it denser add a box of pudding mix to the cake mix and then bake as directed on the package.