Shugenja were nearly always of the samurai caste. Those non-samurai who showed potential in the magical arts were generally either taken in by the Brotherhood of Shinsei and became monks, or else were adopted by one of the clans and made samurai. Shugenja were those that could speak with the kami. They were the priests of the kami no michi religion. 
Monks were not samurai. They occupied their own place in the Celestial Order. Most monks were members of the Brotherhood of Shinsei and were the Empire's guides in the religion of Shinseism.