Haha, I thought psatkha would answer you.
I don't think anyone would object to borrowing models, as long as you credit their source when you publish the mod.
Ideally, it would be even better to contact the authors. There are two points here: you not only ask for permission to use the models but also let them know about your existence. As a result, the model author might mention you somehow, which would increase the mod's popularity.
By the way, the models extracted from the sixth game belong to a user named Zahar; he seems to be on Google+. You can talk to him.