Which LED was flashing red? Was it the one over on the right by the headphone socket, or one or both of the ones over on the left, above the gain knobs? If it was over on the right, don't worry unless you're listening over headphones (risk of hearing loss).
Your microphones, with their 23 mV/Pa sensitivity, could definitely drive a Shure FP24 (a/k/a Sound Devices MixPre) into clipping--though if I've done my calculations correctly, the SPL at the microphone would have to be ca. 121 dB, which is very loud. Still, at close range in front of certain instruments, amplifiers or voices, that's entirely possible. I've recorded opera singers who reached about 122-123 dB SPL at about a 1-foot miking distance, and if you go even closer, the SPL rises rapidly.