YHM stuff is lower-end gear. It's still good from what I hear, although the older stuff had some serious issues with out of spec rails. A lot of guys that use it swear by it. YHM has done a lot of work recently to improve their stuff, to include some mechanical anti-rotation for their free float stuff.
Randall Rausch uses a lot of YHM stuff on his builds, and if it were crap I don't think that he would sell it.
IIRC, the 7" rails fit the carbine, and the 12" are for the rifle. The other lengths are so you can have just the amount of rail that you need and no more. If you're going to put rull length rails on, you might as well just by the fully railed unit.
Depending on where you order the rails from if you are paying full retail you are going to be getting close to the cost of the mid and upper tier manufacturers.