You won't find boulders on slopes greater than 45 degrees and the larger ones are generally lower down. I maybe teaching my Grandmother to suck eggs here but If you use 3 sizes of the fake stone shader merged with highest raised set and plugged into a surface shader with slope max set to 50 and fade zone to 5 you'll get a good result. I'm not sure why your stones seem to be balancing on points but play with the pancake/height settings.