I rack whenever I have lees more than about 1/4" thick, or about every 60 days or so, but I've never fermented tea so I don't know how much you'll get in the way of lees.
As far as sweetening, all you have to do is let it ferment out fully. Then, stabilize with sulfite (campden tablets) and sorbate. Dissolve the sorbate and sulfite in some hot water (like 1/2 cup) and add to the carboy. Rack the fermented tea into it, and wait a few days. After that, you can sweeten to taste. The easiest way to do it is to pull out a sample. Then, sweeten that sample to how you like it and take the SG of it. Then, sweeten the whole batch to that SG. It seems to taste sweeter after being bottled a while, so you may want to sweeten a bit less than you really want it.
You can sweeten with anything you'd like- juice concentrate, honey, a simple sugar syrup, etc.
If you sweeten, you have to stabilize the tea so you don't have bottle bombs. If you do that, you won't be able to bottle carb.
I'd use wine yeast for this, but like I said, I've never made it. I use wine yeast for hard lemonade, though.