Upper Levels Grids under coroplast???

I am building my first C & C cage. The first level is storage 2x4, second level is for Juliet (my piggie) 2x4, third level 1x4 for Juliet, and then a fourth (also with a full coro bottom) 1x4(x2grids tall) for my green cheek conure, Jazz. The vet said it was safe to put them like that as they will have no contact anyway with the floor in between them. My question is, on that second floor (Juliet's bottom floor) do I need grids under every section of coroplast or can I do every other one? Is the coro (4mm) strong enough to hold the piggies (I will eventually have another) on its own??