Hi Lumir,
NS7a1 is the developing stage and is not proper for production.
We must wait at least NS beta1 for this.
You can use NS 6.7 final for production but I’m not sure how easy will be the upgrade to NS 7 and if will be possible because of the major changes of the NS 7. @davidep and @giacomo can tell more about that.
AFAIK, with NS 6.7 final, @Nas made an upgrade for SOGo to the last version and has worked. You can find more about this, here:
SOGo 3 is released
Gabriel