You are doing good.

Nakita is acting like my Bella does with the foster kittens.
I don't think you'll arrive at a fur fight, as adults don't tend to beat up kittens that bad.
The slower the better with integration, have you tried a playpen (cage) for the kitten in plain sight, in the middle of the room?
You can give Nakita all the attention, while she doesn't feel threatened by Phoenix coming too close. Don't worry about the baby, nothing really offends them, they are very resilient. You can give her affection, in another room, away from Miss Nakita.

They will come around, it usually takes mine a couple weeks to relax around new kittens.