Rocky and sandy substrate are the most stable foundation to build any structure on!
Clay soils are reactive so when moisture is added they swell and cause movement. (Which you don't want). Rocky areas and sand don't really get effected.
Pier isolation is a personal preference on how sensitive your equipment is.
A 600d x400w x400L deep hole is a minimum foundation but you can change this volume horizontal so it's only 400d. with the same volume as long as it's min 100mm into natural soil.
Bolt your pier in after as you can adjust it with shims if you get any movement!
Ramset have some good over the counter gear.
Hope this helps
