I have run a set of stock '99 Miata wheels on my 2006. Miatas had both 14" and 15" rims that looked similar in that year -- mine were 14", and I had to grind a tiny bit off the calipers. (Rounded off the square corners on the bit that spans the outside of the brake pad "holders").
I also have a set of Konig Helium rims I run on my Miata that fit fine on the Scion with 205/50-15s. They're 6.5" wide with 38mm offset. The optional 18" rims for the xB with 215/35-28s also fit fine on the Miata, but they're so heavy they ruin the handling and performance.