Do they work well on basic steels like 1095 or 420, or is it like trying to sharpen a stick of butter on a cinder block? If so what grits for softer steels.....I'm thinking fine and or extra fine, but I have never used any of them. I normally use oil stones but want a small portable sharpener that need no oil or water.

The hone is a cutting tool and the aggressiveness of the cut is relative to the grit. Problem with cutting really soft steels is the hone may load up, especially if one uses too much force - heck, too much force is bad regardless of the steel. I can't imagine a properly heat treated 1095 will be a problem and good 420 should also be fine. Only way to really know is to try one.

I would pick the fine for field use where I prefer a more toothy edge but that's just me and others will disagree.

The DMT will work fine on those steels. For a field sharpener, I would get a diafold double sided model. Either course/fine or fine/extra fine.

Yes, it will work "fine", but a Norton Crystolon medium/fine will kick its azz all week and twice on Sunday...