Last night we played an AfroCuban rumba de cajones with bass, second, and quinto cajones (3 players) plus one conga to the side of each cajon. The audience was about 150 people (about 50 standing) in a room that normally holds about 100 and the console was at the rear of the room. The PA was set up mainly for the vocals but I put SM57s on the cajones. I didn't need the mics on the cajones at all, I turned the faders down on the cajon mics and kept just the vocals on the PA. It was plenty loud and I got lots of complements on the sound. The key was highly disciplined and talented AfroCuban drummers who knew how to play cajon and knew about dynamics and not playing over the singers. There were no other instruments in that set. With other instruments (bass, piano, cowbell, claves, guiro, tres guitar, flute, trombone) later on, I just turned up the cajon mic faders a little and it sounded fine. The SM57s sound good on most hand percussion instruments, including congas, bongo, bata, as well as cajon. This is for AfroCuban rumba cajones built from Home Depot plywood, I don't have any experience with Peruvian or Spanish cajones, or with commercial cajones where you are trying to emulate a kick/snare sound.
You might try calling up the cajon players and asking them what kind of mics they have liked to use in the past for a venue the size you are working.