I suppose you can soak for as long as you want. I never had any issues doing the 24 hr soak.
Everyone wants to do it their own way. Of course notching/nicking is always beneficial if you want a faster germination.
No, not all cannas produce seed. More correctly, they may produce the seed pod. It may not contain seed. That, in my experience, is a maturity issue.
Seed pkg size I suppose depends on where you order your seed from. I was/am always able to order smaller pkgs of seed if I was not growing a ton of certain flowers.