Many stage mics have a proximity effect, where the voice sounds more warm and full if you are up close. The general rule is to stay within a couple of inches of the microphone. You should sing at whatever volume is best for you to put the songs across. Some songs are better for quiet voice, some are better for bellowing it out, and there are songs everywhere in between. If there is someone running the PA, they should be balancing the voices and the instruments. If they are not doing their job, or if they are bad at it, there's nothing you can do to save the sound anyway. Here's a hint: make friends with the sound person before you start playing. Introduce yourself and shake her hand. Compliment him on what you are hearing. Ask for her advice. Generally let him know that he is an important part of what is happening, and is a valued part of your show.

Just out of curiosity, why are you plugging in your guitars? I assume the venue has enough mics on the stage, and I have rarely heard a plugged-in guitar that sounds even half as nice as an acoustic guitar through a microphone. And for a two song set, plugging in takes longer and is more hassle. Electronics in acoustic instruments are nice to have when you need them, but that doesn't mean you have to use them every time. Perhaps you should seek the sound person's advice about this as well.

John