Some vets (and animal rescues) advocate for early spay/neuter, and will desex cats as soon as they are over 2 pounds. Generally though you just want to shoot for before six months. Some cats apparently come to sexual maturity even earlier than that, so that's something to keep in mind. Short answer: before she is six months old
As for the declawing, I also have to voice my disapproval. You've been somewhat bombarded with feedback on this issue, and I don't want to overwhelm or offend you--I truly am trying to be helpful. I hope you'll consider what I have to say.
Your resident cat is obviously well-adjusted and relatively unharmed from the procedure--otherwise I don't think you would be considering it. Some cats, like yours, are able to cope with being declawed just fine (after the initial recovery, that is). However, not all cats react so well
. Declawing a cat can lead to litter box issues, as the digging motion is painful after surgery and thus the cat associates her box with pain. I have also heard of declawed cats having issues with biting
(cats get insecure without their claws and will resort to their only other defense: teeth). On the other hand, some cats become skittish and shy when they're declawed. A cat's claws (or toes, rather) are a very important part of who she is--she is meant to climb things, and grasp her prey (i.e. her toys). When you take away something from her that is essential to her nature it can be quite traumatizing
I think that in the case of this kitten particularly--given her background and what you saved her from--declawing could create some real problems.
I hope that you and your wife will consider trying soft paws, like Frostpaw suggested. That would level the playing field just as well as getting her the surgery would. You could also just clip her claws like we do. I really don't think that this kitten will hurt your declawed cat--when cats play with each other it's really all about the biting
our cats play pretty rough and their claws are never