I was planning to use the Wilton separator plates instead of cardboard cake rounds between my stacked tiers. Once the cake bakes, it shrinks up enough so that the cake is much smaller than the plate. I'm worried that I will have to force a big border around each tier in order to hide it. Any suggestions? What do you all do if you use a standard plate and the cake seems too small in comparison?

I think you'll be fine, but if you're worried you could always use the next size (or odd size) smaller plate. You won't be icing your cake on the plate, you'll still have it on its own circle to transport to the plate so the plate won't get knife cuts all over it. You'll basically be using the plate for support, much like the SPS, so if you don't want it to show, it won't hurt if the cake is slightly larger than the plate you choose. After baking (even if it shrinks), and after its iced, you'll be able to tell which plate size works best for you. If you choose the smaller size, your border will most likely only need to hide the cardboard circle (depending on how you ice).