I love your fabrics, Pamela! And your layout plan! I don't think you need sashing if you are alternating blocks... but you'll need to make that call when you get your blocks back.
You can always say, "please match the peach or yellow, or use black or white... but no green or gray." You can get as specific as you want, but be prepared that not everyone will follow instructions to the letter.
That's going to make a beautiful quilt!