If you plan on using the DBX 266 for protection, at least use one side on the crossover high output and one side on the low output. Using it off the main mix sounds bad, kick drum hits at threshold limit vocals, vocals hitting threshold, squashes bass and kick, etc. etc.
Running your system in mono has nothing to do with amps being bridged.
You are planning to have two high speaker amps (speakers are not FOH, which stands for “front of house”, a location you seldom put speakers at) and two low (“sub”) amps, so splitting them left and right could be done whether they are bridged mono or dual mono.