The "wall thickness" issue is for using these "moly" anchors in
various types of walls. Typically, for modern dry wall it is 1/2" or
sometimes 3/8". For things like cored doors, which are 1/8" masonite,
you use smaller ones.
The lath/plaster, is going to be 1/2" or so, I would think. I've never
pounded in the ones you describe, but drill a hole -- typically 1/4",
and then use a hammer/mallet to coax them in. If you have trouble with
them turning, as you tighten down the screw, then there is a little
tool that engages the two notches on the flange of the moly. NO idea
where I picked that tool up, but in a pinch, a pair of needle nosed
pliers will do the trick. Then, when you have them in (do NOT over
tighten, or you can sheer the screw off!), then back the screw out
enough to engage the slots in the back of the shelf. You may have to
fool around a bit with the length of the screw to get a nice fit to
the wall. It is very important to get the EXACT distance between the
holes. as there isn't much "slop" to the fit. Also, you need to make
sure that the holes are level. I'd use a long carpenter's level to
pick the level off of the first hole to find the second one.
I also find it a good idea to drill a 1/16" pilot hole first, as it is
always possible that the spot you picked is NOT between the studs. If
you find that there is something behind the pilot hole (the drill will
not continue to cut after about 1/2"), then you can use an ordinary
wood screw instead of the moly bolt.