Given that you're an early-stage startup, I'd suggest you go with a combination that allows you to develop your product quickly. If django-nonrel isn't seeing much development anymore, it would be safest to move away from it.
Quickest (and easiest) to MVP would probably be Postgres but what when we scale up? I know Instagram did it, but I also read that Systrom went mad making calls to the best minds in the Valley during their Android launch while he was at it!