I do it in a similar manner to what BluePhaze suggested, except that I use individual frames rather than a strip. Set the animation to be 0 fps and upon object creation a random frame is selected via: round(random(0,x)) where x is the number of frames/objects -1.
Here is a basic example I whipped up:
Another nifty command that can be used in a similar context is the choose command. Upon triggering one event you could randomly choose between the first three objects : choose(0,1,2) and upon another triggered event choose between the last 3: choose(3,4,5)