Hi, I am making a birthday cake this weekend and to practice I would like to do a tiered cake with a gap in the middle. I will be using 6" & 10" square cakes. Does anyone know what size separator plates I should buy? Searching on the internet, I can see for the 6", I will need to buy an 8" separator plate but what do I use for the bottom cake?
If you are using SPS, you don't actually use a bottom plate. So you would just need the 4 legs and the top tier plate.
Why do you want an 8" plate?
If you are using Wilton, you have to get the bottom plate. I wold just get a 6 and a 10. But if you are going to get an 8 for the top, then I'd get a 12 for the bottom so that they are even. Don't forget the legs!
I was going to get the 8" plate for the 6" cake because I thought I read somewhere that the plate had to be 2" larger than the cake. I also thought I needed a plate on top of the 8" cake for the legs to stick on. Have I got it all wrong? Should I just buy a 6" plate for the top 6" cake and not bother buying a plate for the bottom cake, just use the pillars with dowels?
There are a couple of ways you can go about this.
1. Use the SPS system. You will need 1 plate for the top cake and 4 columns that will go directly into the bottom cake. The bottom cake will be on any board/platform of your choice.
2. Use the Wilton "push-in pillars". Again, you will need 1 plate for the top cake and 4 pillars. The bottom cake is on a platform of your choice.
3. Use the Wilton 2-plate, column method. This is where you need 4 columns and 2 plates of the SAME SIZE. One plate goes upside down on the bottom cake (little feet sticking up) The columns go on the little feet. The top cake plate goes into the tops of the columns. The bottom cake goes on a platform of your choice.
Method #3 is the one I would least choose. Method#2 is ok, but you need to make sure you put the columns in only once. If you take them in and out of your cake, you make the holes bigger and there is less support for the pillars. Method #1 is the go-to system! 
As far as the size of the plate for the top cake.. You can use a 7" or 8" plate if you need room for a border. You can use a 6" plate if you are doing a smooth edge.
HTH
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