Ok, theres a pic that im rendering, and it has 2 problems with it.

1. The mirror that i'm using reflects stuff, thankfully, but it also has a very bright "shine" to it. Thus, anything that is reflected is almost overpowered, and the mirror almost looks like its glowing.
   what Surface settings do i need to get a "flat" mirror that isnt so bright?

2. When saving as a PNG file, i try putting the pic together in Photoshop.
   Unfortunately, whatever is layered onto another image has some sort of white line outlining it.
   how do i get rid of it/fix it?

will be attaching example pics soon.

1. Adjust the specular settings - it sounds as if Glossiness is too low and/or Specular strength is too high. Tweaking how the lights hit the mirror surface might also help.

2. Render against something close to the final colour the items will be composited over, or use the defringing/remove White matte commands in Photoshop.

As a rule of thumbs: Pose over a WHITE background, render over a BLACK background!

And before rendering, change the viewport to "cartoon shaded", that makes it easier to find pokethroughs and overlapping, even with transparent props and clothes.