With the more information you've given in your last post, here's my recommendations:
1) Excavate at least 12", and put in at least 6 inches of heavily compacted, well graded crushed rock aggregate (gravel). Since you have freeze/thaw issues, it is even more important to have a good base. 2 inches of used/contaminated rock is definitely not enough to do what you need. I would consider going to more rock and less concrete. 4 to 5 inches of concrete is fine, as long as you don't make it 3 1/2 inches to use a 2x4 form. Use a 2x4, but prepare it so you get a minimum of 4 inches. Or, there's nothing wrong with your plan of 6 inch concrete, but you need a minimum of 6 inches of compacted rock under it.
2) Definitely put in reinforcing. Mesh is as good as rebar, as long as you block it up in a positive fashion so that it is in the middle of the slab. Buy a bunch of concrete block, and break them up with a hammer into pieces that you use to wedge up the mesh. DO NOT RELY ON PULLING THE MESH UP AFTER THE CONCRETE IS PLACED. It doesn't work. #3 rebar is quite cheap, usually I use it 18" on center, and tie mesh onto it, and then block the whole thing up 2 inches to fall in the middle of the slab. The rebar and mesh does add some strength, but more importantly, when the concrete cracks (and it will), the reinforcement holds it together so it doesn't displace.
3) DO NOT DOWEL INTO YOUR GARAGE SLAB. If you get a problem occurring in the slab outside, you don't want it to displace and/crack the slab inside. You'd much rather have the slab outside move independently, and hopefully return, than move, break the garage slab up, and then have both of them screwed up and hooked together.
4) Definitely saw cut the joints if you can instead of tooling the joints. If your contractor friend has a correct joint scoring tool, you can use it. But, that works a lot better for sidewalks than it does for slabs. Using a trowel and edger can work, but not well at all. Hard to score deep enough to be effective in making it crack where you want it to, particularly if you have rebar.
5) Do not put a vapor barrier under the slab. Will not serve any useful purpose.
6) Do not rely on a large flat plate compactor to compact your gravel. Wet it thoroughly and use wacker packer/jumping jack type compactor to thoroughly get the gravel compacted.
7) Use air entrained concrete. Specify 5 to 6.5% air entrainment. The concrete company knows how to do that. That will help keep the concrete from spalling after water gets onto the concrete an it freezes. Essentially, the air entrainment leaves some void space that water can expand into when it freezes, so it doesn't break the concrete.
8) Work the surface a minimum to get it smooth enough to finish. I'd finish it with a smooth broom finish, going in the direction of the water flow/slope. If you overfinish the concrete, working up a "cream" and repeatedly troweling that, you create a layer that water gets under, and when it freezes, it spalls off the surface. Overworking also removes the air entrainment, and that increases the spalling potential. For slabs and roads, the minimum you can work the surface and still finish it is the best and most durable surface.
9) Slope the concrete a minimum of 1/8" per foot for a hard troweled, flat finish. 1/4" per foot is preferred if you use a broom finish. Less than that and water will pond on the surface. I'd recommend a broom finish instead of a hard troweled finish, as it will be more durable and less susceptible to spalling in freeze/thaw cycling. Hard troweled will be easier to roll over with a creeper, and will be easier on your knees if you kneel on the slabe. Both have their benefits and detriments. Chose whichever works best for you.
