Hello,
I also have a hamster with this problem and i think that they are susceptible to this because of bad breeding and a poor diet when they are young, as harry is also quite a weak hamster with not very much muscle mass, he has dislocated one of his front legs before, hair loss etc, the vet actually suggested that all of these things could be symptoms of cushings at one point. Harry actually lost most of his nails (as far as i could see) at one point and i know that it wasn't down to the diet i was feeding him as i had other hamsters on the same diet. I have seen some improvement since changing to the Supreme Science Selective hamster food and i have been feeding alot of baby food with proteins such as the chicken dinner ones and he has put on weight since and his nails seem to be a bit better. You could also try Pedigree milk bones?
I'm going back to the vets this afternoon with Harry, so if they can suggest anything else then i'll let you know.
Well done for adopting cooper! Hope he is settling into his new home well