Random clouds with extra properties

Hi all

quick question for anyone who may know.

I am designing and preparing a scene, in which random out put will be required. I would like the clouds to be randomly generated from a symbol in my library, i.e a ready prepared vector done in illustrator.

I need this cloud to be randomly generated from left to right and a varied size, however I would only really like three clouds on the screen at any one time, but different in size and position within my sky area.

Also different speeds, can anyone give me some tips, help in achiving such and effect?

Many thanks for any contribution.

Trev