Hi AMS2007,
Yes it is true that by surrounding the pmos with an nwell ring connected to power line and surrounding the nmos with substrate ring connected to ground line will give you a good isolation for both transistors...but in general this rings are not much of use because as research showed. The stray electrons and holes(minority carriers) travel deeply into the substrate and are not properly collected by the nwell and substrate ring. Never the less putting these guardring is better than leaving it open.
Cheers