You have a gorgeous little flock and it's great you were able to give Kookie a second chance at a happy and fulfilling life by rescuing him.
Kookie's behaviour is perfectly understandable given what he has gone through so early on in his life.
For how long have you had Kookie? Did you take him to the vet for a wellness check and to possibly have a better idea on the extent of the leg/foot injury?
It would be good to know the condition the leg/foot is and if Kookie is in pain from using the leg or not.
From the photo you have posted, he seems to be able to have some grip on his bad foot and that in itself is good, that despite the injury he suffered he can still use the leg/foot for support.
For Kookie's case, you can adapt the cage to better suit his needs, you can place ramps, flat platform perches for him to better rest his feet when required.
If he has the tendency to fall off a perch, you can cushion the cage's bottom with a towel or t-shirt and place the cage liner on top of it.
When having out of cage time, you can limit his flight range by confining him in an specific bird-friendly area of the room for extra safety.
Birds are amazing in the way they can recover and adjust after going through a tragic event, after facing very difficult challenges which in some cases have changed their lives forever.
And with the needed love, understanding, dedication and support of their owners, a whole lot can be accomplished for the bird's welfare.
Despite the challenges that a chronic health condition or a disability can bring, with the right care it's perfectly possible for a special needs bird to lead a happy and as normal of a life as possible.
I currently have two birds considered special needs, one of them, my lovebird Sindel has a more serious physical disability, she is missing a good portion of her leg that had to be amputated to save her life, she was 5 years old at the time.
The way she coped during the long recovery process and how fast she adjusted to her new reality was something incredible and truly inspiring.
While Sindel had my full support as well as her mate's, I found myself drawing strength and confidence from her and that allowed me to go the extra mile and do my very best and go beyond what I considered to be my limits to further help her.
One year has already gone by and she continues to be a happy, healthy and well-adjusted lovebird, who happens to just have one shortened leg.
Sindel is not defined by her disability or on what she can't do because the fact is she can do everything she sets her mind to, only she does these things in different and equally accepted ways.
If you want to, you can check this link and there you will find out more about Sindel as well as Khaleesi: https://talkbudgies.com/your-budgies-...adversity.html
I sincerely hope Kookie's condition improves and you are able to help him overcome the challenges.
Best of luck.