No it's not that, The SM58 is like the industry standard because of it's lack of feedback problems. The 57 is better for impact sounds like snare, guitar cabs and female vocals,,,the 58 is a great mic adding depth, clarity and an improved bottom end. The most gain you can get basically, is just before it feedbacks...then, drop the master volume back slightly. Eq on the main front of house system can cut out rogue frequencies. Also, make sure you are not too close to floor monitors or FOH speakers. Then I would ease in the compressor until you can feel the effect. Of course, too much gain will increase the likeliehood of feedback so just play gentle with it. Do you have a V.U. to see if and when the gain is clipping?
When gigging locally, i use a nice mackie head with built in compressor and I apply a sprinkel of reverb (med hall). On a full band 3K rig with outboard desk and 2 monitor mixes, I would chain in a compressor through an aux send and apply accordingly on the appropriate channel. It all depends what your going for at the end of the day but be careful not to overcompress or you may lose dynamics in your vocal. It is a great effect but I use it more in the studio than live. Mic technique is a crucial factor, it;s about giving the mic the best signal possible which normally means having the mouth cloase to the mic but back off when the high note is a gonna come or slighlty turn the head left or right. My vocals are all over the range, Sting to Gil Scot Heron, Macca to Marvin....it will just come natural in time but moderation is the key. Don't rely too much on effects for your singing.
Right I'm ramblin...just done 2 gigs today and am bushed. Hope that helps some.