Would an Iphone 6 Plus case fit an Iphone S6?

Would an Iphone 6 Plus case fit an Iphone S6? - 1

No, it won't

iPhone 6 Plus Dimensions - 158.1 x 77.8 x 7.1 mm (6.22 x 3.06 x 0.28 in)
iPhone 6s Dimensions - 138.3 x 67.1 x 7.1 mm (5.44 x 2.64 x 0.28 in)
http://www.gsmarena.com/...hone1=6665

iPhone 6s is smaller than iPhone 6 Plus. Better to buy iPhone 6s case separately
https://www.amazon.com/...6-Cases/b/

Probably not

No. The 6s is smaller than the 6 plus.