Okay, here's the deal. This is unofficial, but reliable information.

There are no source files, and there won't be any.
The creature limit has been overcome. It's possible to load a DEF file by specifying its name in a task. In essence, 42 creatures are sufficient because there can't be more slots in battle. And outside of battle, there are even fewer (for the hero).
This is done through the UN:C function. Details can be found on df2. WoG --> Scripts

Mephit is not finished, and I haven't seen Bisika anymore.