Hello!  We were in a similar situation, although I got Bizkit from the shelter as her foster when she was 11 months old.  Same situation, older rear leg injury, not using leg much at all.  We are now two weeks post amputation and she is doing really great.  She had zero trouble adjusting, probably because she had already adjusted.  The vet said she has zero limitations now in terms of playing, walking, exercise, but we are taking her to a rehab in a couple weeks to get a specialist opinion and making sure we don't tire her out with too long of walks or exercise.  She gets around just fine.  

She's 29 lbs but kind of tall, and has not needed a ramp.  Pre-surgery and post surgery for 2 weeks she was not allowed to jump up on couches, beds, etc.  And we still kind of watch it with cars so she doesn't strain anything and just lift her in.

As Mischief says, our main issue was energy/activity control.  Before the surgery so she didn't hurt herself, and during those two weeks after in recovery.  It required medication, separation from my older dog, and LOTS of non physical stimulation such as puzzles, frozen toys, other enrichment activities, training.  

One thing that is beautiful to see now is how much more settled/settle-able she is now.  Now we know that it was causing her pain.  I don't know if over-activity is a symptom of pain in younger dogs but it seems to have been with her. 

Our best purchase post-surgery was a soft cone, she could sleep in it easier and stopped bonking into everything, but she did still need supervision in it as every now and then she would try to lick "through" it and I didn't want the cone rubbing on the incision.  I also ended up buying lick mats to freeze, some "everlasting" treats, pulling out my old dog's puzzles and toys, and looking up ideas to use her energy without being able to play or go on walks. 

Thank you for rescuing Cleo!!!