Cats and fish oils?

I have two cats, one is 15 i found him behind my friends house when i was in the 10th grade, fed him through an eye dropper he was so young. he has been diabetic for the last 5 years so i give him insulin with every meal, he was a bit heavy but has lost some weight since then. other than that he is very healthy and happy. he is treated like a king, trust me. the vet said there is a possible correlation of no mothers milk as a kitten to the diabetes, when i found him his eyes were barely open.

the other i stole from a tenant on my parents property, idiot bought him from the humane society but didnt take care of him to my standards. he would let the cat out at night and i would often see the kitty at the door step late at night begging to get in. i once asked him what he fed the cat and he said scraps..i almost punched him, he would also have a water dish outside but it would always be dry, i would go to my parents house to feed the kitty then i just took him and brought him home, he is like 5 im guessing.

would they benefit from taking fish oils? the older one has slowed down a bit but is still very vocal.