at the sizes you list they are still juvenile fish-when they reach about 10-12" then their adults. There has been assorted reports of xanthichthys triggers spawning in large public aquaria. However i have not heard nor read it being done in the home aquarium.
Not sure about blue cheek specifically, but one can extrapolate that these fish have a similar type breeding pattern; males nest build(dig out shallow depression), attract females, mate w/ multiple females and males guard eggs. Many report these fish are haremic, i suspect huge tank sizes are a must