Regarding the weight that all depends on what is healthy for your dog. See what your Vet says :)
For my Rottweiler with HD I fed grain free food as grains can aggravate arthritis. Then I found kibble was not very good for her then switched to complete grain free canned, and then grain free canned mixed with freeze dried and or dehydrated. Then I moved to complete home prepared and even tho she now get grains in her diet she seems to be doing the same if not a bit better. I make sure she gets lots of raw cartlidge which contains natural glucosamines. I give her a glucosamine supplement that contains Glucosamine, MSM, chondroitin sulfate, Vitamin C etc. Green lipped mussel is great as well. I too give my Rottweiler sardines for omega 3's and steamed salmon, herring has a good amount of omega 3's as well.
I keep my Rottie's exercise consistant and never over do it. We walk mainly on old country trails so they are soft and natural and we never walk on pavement. Not that this is any great deal but the surface seems to be agreeing with her. We also do lots of swimming and I learned about acupressure to relieve pressure points.
Athena was diagnose with very bad degree HD at 1 1/2 and was re-X rayed at a bit over 2 years. She was in pretty rough shape back then and had a hard time getting out of bed and stairs etc. Since I learned about proper foods, holistic and herbs to give her and homeopathy she is now soon to be 9 years of age and is doing so well now you would not know she has HD or that she had ever suffered at any time. I am so proud :)
Here is a great site for you to review as well.
Arthritis