After Effects opacity/transparency/alpha

Two questions-

  1. I’m applying some animated effects to text. I apply the effects to the text using silhouette alpha (different layers). My question is how to make the black background either a) transparent or b) an image that isn’t effected by the effects layer. I hope you understand what I mean.

  2. What is the best way to AE clips in flash? I know sequences of jpgs lag flash like hell, so would I be better off using small movie clips? What would you suggest?