Edit: Missed your middle post.
For pipe under the floor, the more thermal mass the better. It will even out the hotspots. Just make sure you have a much higher R-value below the pipes. I generally use reflective insulation plus fiberglass, the pex will be in a small airspace. R30 is cheap.
There's quite a bit to this if you haven't done it before. Is this your sole source of heat, or just trying to make the floors more comfortable? It can be difficult to get enough BTUs out of a floor, especially if the home is not very efficient.