Kamusari was just a technique Roger developed (or learned, not sure if it's a Gol Family or a D thing)... Rayleigh developed/learned his own style differently. Shanks was Roger's apprentice, so he naturally learned his mentor's technique.
If Rayleigh parallels Zoro in his fighting techniques too, he shall have an unique style. Despite Zoro having two different masters (Koushirou and Mihawk), he apparently didn't learn a single technique from them (besides holding a sword and haki)... he developed his own style.