Can I rent the fridge after the move in? I'm having trouble reaching my roommate regarding the issue of a fridge.

Just order it. And if they refuse to pay you half of it, just put a huge padlock over the fridge and deny him/her access.

I'm more worried about the fact that he ordered one too.

Well you could always wait until after September to see if your roomie ordered one and if not order one then. Another option is to call Coldex directly with your inquiry and find out what would happen if that's the case (that both you and your roommate ordered one), such as if you would get a refund on one of the fridges.

Contact page: http://www.coldexrents.com/contact.htm