Good for you for taking in this cat!

It is ok to "fix" your cat at most any age.
As for the declawing, you probably shouldn't, and as it was said before, a vet probably won't. BUT, the good news is -- if the claws get to be trouble, your vet can show you how to trim the claws properly and you can also opt to get these plastic tips that go over the claws.
Post a pic of the kitty?