I've had cats all my life.
The last one I had, though NOT to certified was my emotional & mental support pet, especially when I suffered from Depression for months. She was the only reason I didn't kill myself. She always knew when I needed a laugh (and would stick out her tongue or just be silly in some other way. I'm sure certain like with determining which dogs would be a good idea for a seeing eye dog (and I know part has to do with personality) that I'm sure that may be true as well for being service 'animals'. A cat can be just as loving and loyal as a dog. I'm pretty sure that cat Tara (I believe was its name) that saved the kid from the dog that attacked him would agree.
I believe that whether the person already owns a particular animal as a pet, that it should be able to be trained as a service animal. Get a dog and a cat and train them for a diabetes spike and be able to call for help etc. epilepsy, anxiety and esp. panic attacks (I really hate those).
Cats take up less room, are more quiet. This is good for apartments, (where dogs may be too big) . So broadening the animals that are trained would be a great idea. Not everyone can have a dog, or a cat