This thread ought to be called "SCOBY Do"
When I did it, I let the commercial kombucha come to room temperature and a tiny scoby had started to form, but I just added the bottle's entire contents (16 oz.) to the 32 oz. of tea. The 2:1 ratio of sweet tea to starter was enough to defeat any "baddies" that might have been lurking. DR's got a point though - if you can culture a more robust SCOBY in the bottle it can't hurt.
DR did you grow your bottle SCOBY topless? Sorry, did you take the lid off?
+1 plain white sugar (though I do use organic white)
One thing I find very weird - commercial kombucha in plastic bottles.
No... I just let the bottle set at room temperature. It was sort of accidental with the mango one. My dad got it for my mom and she didn't like it. She left it setting on the counter and when I got to it, there was a scoby on top. Then I bought a bottle of GT's original, and it had a floaty in it. I decided not to finish the bottle and instead just let the scoby grow.
I've never seen commercial kombucha in plastic bottles. Course, all I've ever seen around here is GT's.
SCOBY SCOBY do! Should've been more inventive with the name
Thanks guys. This is all really helpful!!!