This is what I would suggest to you.
Your SM57’s are great for guitar cabinets.
Your SM58 is great for Vocal. (givin that you have feedback worries)
If not then look at a large condensor mic. For cheap I had good luck with the Shure KSM27 ($300) that works great for Acoustic Guitar or Vocal. I’ve used it in live setting on Acoustic Guitar and had great success. Also used it in the VO booth with great success.
As far as a mixer I’d stay away from Mackie (personally I’m not a fan) look into Yamaha’s mixer’s or Allen & Heath those two are pretty good cheapies!
Adressing your “Hiss” Problems coming from the SM57’s. Well first guess would be your gain staging. Try bringing your Channel Fader up to Zero then tweaking your Gain (at the top of the channel strip) until you are getting a good level on your VU’s and, most importantly, it sounds good not muddy and not distorting. Of coarse this is with your master fader set to Zero.
I’d stay away from the Lav’s for the singer or guitar. Those two sources would put out too much SPL for a little Lav. Keep using your Lav’s for Interviews.
Hope this helps you out and isn’t too late. The way I look at it is Any knowledge is never to late.
Brian B–
Avid|DS Artist–
Protools Editor–
Live Sound FOH/Monitor Mixer