Glen,

You should sulphite as soon as the fermentation ends (which would be <
SG 1.000). Sulphiting during fermentation is pointless (and potentially
harmful) since all the SO2 added will simply bind to acetaldehyde. I
recommend you also give the must a decent dose of SO2. Of course, there
will be a small window of opportunity for MLB during fermentation (when
SO2 levels are very low due to binding & "blow-off") and you can try
and control them with a low temperature (wouldn't you be doing a cool
ferment with Gewurz and Riesling anyway?).

HTH,
Ben

Improved Winemaking
http://members.tripod.com/~BRotter/MLF.htm