As for the left or right... it's a stereo mixer. You can say send a channel to 1-2, assign 1 to left and 2 to right, and then the pan control on the strip will move the source from left to right. If you set both left and right on a given sub fader, then it treats it as a mono signal instead. You can also choose to send them to both left and right. So, you could put the voices into 1-2, and assign both 1 and 2 to both left and right, so all voices are in mono and come out of the centre of the rig, while putting all the other instruments into 3-4, assign 3 to left and 4 to right, and set up stereo imaging for the music.

'Course, most of the time that's not going to matter... but if one were using it to record a live show it might be nice to have it come out that way on the tape.

sm58s... main reason I'm interested in those is because they're relatively cheap, can take a beating, and are readily found and replaced if needed, meaning that one will end up with consistent mic performance across the rig, which will make EQing a lot easier.