Please don't declaw the new cat. I assume you know the problems with phantom pain and the behavior problems declawed cats can have-after the pain of having their knuckles removed. You can introduce the cats slowly, give them time, and if they fight, it would be better for the new kitty to go to a home without declawed cats.
The worst time would be the introductory period. I have had cats that did not get along, and I have seen hissing and fighting, but have seen one get hurt. However, a bully cat might be able to take advantage of a docile cat with no claws. Watch them closely before making a decision about finding another home. You're very kind to take an abandoned cat in. I don't understand how his family could leave him!
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A dog, I have always said, is prose; a cat is a poem. ~Jean Burden