How to bake wedding cakes from box cake mix?

i'm baking several 4 tiered wedding cakes, 12", 10", 8", 6" x 2" Rounds from box cake mix. i'm not a real strong baker but I"ve been Decorating for a chain bakery for 8+ yrs and i make bday cakes for my girls 10+years. Question: can i bake different sized pans at the same time. also on the largest pan how do i bake evenly without over-baking the edges if i dont have the baking strips? does it matter how full they are? i've been filling them 3/4's full and cutting off the mound of cake, just wondering if there's another way.