One campden tablet, crushed and dissolved, per gallon of wine is fine! You didn't add sulfites when you racked that kit, that's why it recommends adding some at bottling. You won't oversulfite if you go with the one per gallon.
I'd NOT use the sodium metabisulfite, since it adds sodium to the wine. Some of the campden tablets are are sodium meta, and some are k meta, so check the label. K meta (potassium metabisulfite) is what I use.