What I do is create Cloud Groups and SAVE THEM as VOBS. Then When I'm BUILDING a scene I load in the Cloud Vobs and move them around to fill in spots where they might look good. I'm still in the very EARLY STAGES of creating a nice collection of Cloud Vobs and this is why my Cloud Scenes still LACK the kind of details and REALISM that I really want.

As you can see, I have generally placed four Cloud SPLATS ("Splats" is the term the "Cloud Pro's" use) into each GROUP- which is saved as a VOB. Later on I may go into much more COMPLEX Cloud Groups- probably adding more Vapor Clouds that will be ROTATED at 45 degree angles- this creates much greater Cloud REALISM...