Unfortunately there hasn't been any confirmation you will be able to make a custom party from the start. Considering the amount of people who painstakingly launched four instances of the game to achieve that, it is a bit disappointing Larian did not hinted at a better option during the PFH.
Based on the Interviews it seems the custom characters are specific NPCs in the game you first have to find and, once hired, can be completely customized (at least their class?)
