So I'm doing a 7 in. top tier, can i put the 7 in (obviously on it's own board) on top of a 6 in. plate when using SPS...I'd think yes, I don't want to do a 7 in. b/c my cakes cook down a bit and I don't want the plate sticking out from the cake...but neverh aving used SPS before, wanted to check, I'm sure the experts on here will know.

I'm sure you can since it will be on a cake circle the same size as the 7" tier.

I've used a size smaller in a pinch and it worked out fine.

I've used smaller and in fact prefer it so that I don't have to worry about it sticking out and showing. I usually to 1" smaller in diameter, a 7" plate for a 8" cake.