Still leaves me with that bad feeling about interlinking too many private doms though. Especially from the same registrar.
I'd say you're right. It's not a good idea to interlink, say 100 domains and probably not even 30. (It's probably not even a good idea to try to operate 100 domains.) That kind of interlinking is going to stand out in Google's back end analysis tools like a flashing red beacon that says "I'm trying to boost my rankings."
If interlinking is only 1% of each site's total link profile, then you may have something that will pass the sniff test. But then you wouldn't even think twice about it, you'd just interlink when it makes sense for the visitor.