I saw an article where it says that you can establish two way trust between 2003 domain functionality and 2008, 2012, 2012R2 but i cant find anywhere is it possible to do this with Server 2016. The release date is end of September and our domain is due to an upgrade so need to know if it's possible to make a new forest based on 2016 and establish two way trust with 2003 domain functionality?
Precisely. And in Microsoft's recent move towards more rapid development, some decisions are going to be made literally days before shipping. You really won't get a definitive answer for a product not yet out. Only prerelease info (subject to change) and conjecture. Not good for basing upgrade plans on.