A family can consist of Sprites only, or Background Tiles only, or Particles only, ect. Is that by design?
Yes.
re there plans to extend this so that I can have sprites, and particles in one family.
No, this isn't really possible if you get in to the engine internals. For example, what actions, conditions and expressions do you display for a family with both Particles and Sprites? The ACEs just in common, or all for both? If just those in common, it would remove most of the ACEs making families not very useful. If both, then you end up with weird situations in the event system - if a family has one Sprite picked only, and you run a Particles condition, action or expression on it, what should happen? It's not clear, so it's simply not allowed.
uestion two. I looked up and down the manual for the Family Information section but could not find it.
Families aren't in the manual yet because it's not actually a finished feature yet - we still want to add Family Behaviors, and then write up all the documentation in one go.
(A workaround to 3 objects in 1 family is just have 3 different families for each plugin type, then you at least only need 3x duplication instead of more.)