You may have delay issues. I've tried to do a "on the fly" matrix with a soundboard, fm soundboard feed, and mikes, and anyone of the sources could hit the recorder a few ms before the other resulting in an echoey sound. Thats fine if the sources are recorded on separate tracks, but if you are mixing an fm soundboard feed with the mikes on a single track or two widely spaced mikes to a single track, there may be issues.
Give it a try, I learned a lot from the experience.