I'm confused as to why you would think it'd be different than adopting (not sure if you actually mean buying) your dogs. What's your concerns/questions about it?
But to your other questions:
I don't believe he would spray if he was spayed (which I'm sure he would be before he left the rescue. Maybe someone can correct me if I'm wrong.
I'm sorry, but if you are THAT concerned about the cat scratching, you shouldn't get him. Declawing is the equivalent of rubbing your fingers off from the knuckles. Sound painful?
I have a kitten and she has never hurt my dogs or furniture. She HAS accidentally scratched me once or twice though.
Get a scratching post to encourage natural behavior and you'll be fine.
Maybe if you'd contact the shelter and explain the situation they would hold him for you until you get back?