Here's a thought ... I'd borrow/rent a wireless set-up for your guitar, so you can walk out in front and actually hear whats going on, make sure the main speakers are facing straight out, the first thing to do is EQ and dial in the monitor level, with the mains turned down, the monitor level/sound will be somewhat different than the main sound, I'm not a big fan of the SM-58, the pick-up pattern is too wide, I'd go with the Audix OM-7, it has a tighter pick-up pattern= less feedback, remember try to keep the monitors somewhat behind the microphones, this will minimize the feedback issues, ( I would recommend practicing in getting a good monitor mix first, and this entails learning how to place and EQ the monitor level, so everyone can hear themself's at a realistic level with out feedback, once you master this, the FOH sound is a lot simpler ) as to the main sound your somewhat restricted as power goes, but you can make it work in small clubs, the bigger places you'll need larger amps/speakers ... IMHO