Containers might not be useful for what you're trying to do. As a Container will spawn items in that container if one is used.
What you might want is to create a family, whack a bunch of sprites in it, then give the family a variable (say a number) and then at the start of the layout, randomly give all instances of the family a random number between 1-5. Then if the Sprite variable = 1, spawn object A, = 5 Spawn object B, etc
Let me know if you need a CAPX to get what I mean.