Glynne, in my book you have most definitly rescued these cats.

Your best bet would be for supervised introductions after a day or two. If the friction is really bad, try again in another day or two, gradually increasing the times the cats are all together. Always be there to rescue in case of a really bad fight, but don't be alarmed at a few hisses, growls, and swats. Only step in if the fight is clearly out of control or lopsided.

Forget the screen door, a cat can shred even an aluminum screen in seconds. Besides, they get to know each other by smell - appearance means almost nothing to a cat.

The key is gradual and supervised.

Bless you for taking in these little throwaways.