African Greys are special. If you think Killi is smart, wait until you get to know your Grey. They are amazing. And, they can get into a lot of trouble because of it. I had a TAG living with me for 8 months and I have raised several CAGs. I will have a Timneh of my own someday.
In my experience, my Senegal displays the typical jealousy that Senegals are known for and he does not get along with any other birds. He tried to bully the Timneh when I brought her home and she didn't fight back at first because she was in a new situation and nervous. That didn't last long. They never got along. If I had one on my shoulder, the other would rush over and pick a fight so they could be on my shoulder. Having two birds fighting over you in your face is not pleasant. A lot of this is going to depend on the individual bird. I have a Senegal who will not accept new birds. Yours might be okay with a new bird. Her being female might make a difference.
Keeping a Grey flighted is great because baby Greys are known to be clumsy and clipping their wings only increases that. The Timnehs especially are known to have problems with damaging their keel bones from falls. Remaining flighted should definitely help with that problem. As for behavior problems, I don't know how remaining flighted will effect that. I believe that a lot of the behavior problems in Greys occur because they are so intelligent and we aren't providing them with enough stimulation. They need lots of toys... interactive toys that they have to figure out. Training your Grey should help with that too as that provides plenty of attention and interaction. Also, diet is important. Greys need extra calcium.
Differences between the two. In personality, Timnehs are said to be more adaptable to change and more social with more people. I think that if you properly socialize either and get them used to change at a young age, you shouldn't have too many problems. Some Cags will go to anybody and some Tags are strictly one person birds. They have the same talking ability so that shouldn't be an issue. Willow, the TAG who stayed with me, started out going to anybody and I encouraged that but she did start showing a preference for me around a year old and could be nippy with others at times. She was incredibly smart and funny. She used to give me a look of defiance before tossing a toy on the ground. I'd tell her no and she'd give me that look again and toss another one. She was very active and crawled all over her cage. I would say 30 x 24 or so would be a good size cage for a CAG, 28 x 24 minimum for a TAG. In the end, I say go with which ever bird appeals to you more. Raise him well and provide him with a loving, stimulating environment and you shouldn't have any problems with a CAG or a TAG. I personally prefer the TAG but only because of the size. I think the CAG is prettier.
BTW- Biting is not a typical Grey behavior that I've heard though they are notorious pluckers and can become phobic.
Hope that helps. Let me know if you have any questions I didn't answer.
"I bet the sparrow looks at the parrot and thinks, yes, you can talk, but LISTEN TO YOURSELF!" ~ Jack Handy ~ Deep Thoughts