Any freebies are usually not commercial, but anything I buy is mine to use commercially and I don't have to give credit although I usually do. If I can't use it for commercial renders animations I'm getting a refund. You have the right to do your movie, or comics no matter how big it gets, but like wolf said you can't distribute the models. If they want a change you will have to do it or point them in the right direction to buy the models, which I have done on one project. 
Marque