On the XR18/X18, Channels 1 and 2 are labeled Hi-Z, can I use the Hi-Z inputs for microphones?

Yes, the Hi-Z only applies to the 1/4" TRS part of the Combo jack. The XLR Input is not Hi-Z. The TRS portion is 1 MΩ/2 MΩ, but the XLR portion is 5 kΩ/10 kΩ