Meta and Sorbate will not stop a fermentation, nor will it kill yeast, this is one of the biggest misconceptions I'll bore you with an outline of Meta and Sorbate:
Potassium metabisulfite is used to prevent spoilage and further fermentation by removing oxygen, by removing the oxygen it chokes off micro-organisms and will prevent them from reproducing. It will not stop a fermentation.
The only reason that I'd ever add
Potassium Sorbate, is if I plan on back sweetening wine that has finished fermenting, it does not stop fermentation, but is used to keep wines from starting to ferment again after the fermentation has been completed, it does this by rendering micro-organisms unable to reproduce.
I use to make sparkling wine the traditional way until I found a post on another website posted by Bzac, one of the best winemakers on the net in my opinion, the post can be
found here, a few posts into the discussion Bzac posts his method, I've used this method several times with great success.
I hope that this helps.