To go back to his ravenous appetite, if he appears to be over 10 years old, a total T 4 measurement would be good to obtain. If he was free roaming outdoors, I would just make sure he gets dewormed, even if the fecal flotation test is negative for worms. However, if this started on Megace, it might be the culprit.
As for the kitty interagression, best is to control the problem before it starts. Keeping him away from the other kitty is best right now. The medication can change his behavior, but may not stay that way once the medication has stopped. While on medication, he may realize, well this bunch is not that bad after all! However, cats are normally not pack animals, and may tolerate one another, but some won't. The MEGACE are synthetic progestins, and should not be used long term. There are possibilities of: profound adrenocortical suppression, adrenal atrophy, transient diabetes mellitus,(increased water consumption, increase urination) personality changes (that's what we are after), increased weight, mammary hypertrophy, neoplasias, and liver toxicity possible.
Plan for kitty to kitty aggression:
-Gradual controlled reintroductions using a distance gradient. Reintroduce when positive income is likely (food reward, body harness).
-Bell on aggressor cat.
- Create an environment of plenty, multiple resources in multiple locations (climbing, perching, litters, etc).
-Use a common towel to rub cats each day with the idea of transferring scent.
- Feliway (Abbot) sprayed in environment (or the plug in, one is good for 600 square feet).
-Anti-anxiety medication as needed for fearful or anxious cat. Appropriate medication would be: Fluoxetine (Prozac), Paroxetine, Clomipramine. They need to be used over a month period as a minimum. This situation will not resolve overnight (I know you know this!).
-To have cats tolerate each other; they probably won't be best friends.
- It is critical to address the excessively fearful cat (could be your other kittys).
-Cats can stay aroused after an aggressive event for a long time, may need days to weeks of segregation prior to reintroductions.
Natural products you may also consider would be Nutricalm from Rx Vitamins (order through your primary care veterinarian). Acupuncture and Chinese Herbals according to his examination.
Hope that helps! Let me know if you have more questions, or need additional information.
Hello to the gang!