Re: Plucking - there's a lot of causes and reasons for this behavior. If he has not been to an avian vet for a full work up recently, to include fecal, bloodwork, X-Rays, skin swabs, etc, then I would recommend starting there to rule out medical issues.
Article containing veterinarian's approach to diagnosing and treating feather-destructive disorders
featherpicking.com
After medical things have been ruled out, then you can look into behavioral reasons. Boredom? Stress? Anxiety? Diet?
The feathers may never grow back on the stomach if the feather follicles have been damaged too much.
Re: Biting - This is also kind of complicated... and since he's new to you, he may not trust you and needs space. That's okay! First things first! The only bite that can't be rewarded is the bite that doesn't occur. That is, the more times your bird feels the need to bite, the more ingrained it becomes to bite. Learn to read his body language and back off before a bite occurs. If you do get bit, don't punish, don't ignore. Simply remove his beak from your flesh and turn your back away from him for several seconds.
It will take time to get him comfortable in your home and potentially okay with everyone. Don't force interactions. Target training can go a long ways into teaching desired behaviors and making it easier to get in and out of the cage.
Someone recently told me that there was only one trainer on the internet that had free training advice. This is, quite simply, false. This “trainer” they spoke of is an amateur that has taken advice from salesmen who market themselves as bird trainers. Not to say that they don't enjoy helping...
forums.avianavenue.com
Re: Nutrition - Another hot topic! Many people will agree that variety is best. Most will say that a pellet based diet with fresh foods (primarily veggies), sprouts and some nuts is the way to go.