In a low light situation I messed around with switching the shutter speed to 25 frames per second (not cineframe 25 or anything like that). I loved the extra light I was able to get out of it but what does it do to the image? If eventually I want to convert to 24P, what havoc would I wreck by switching the shutter speed to 25? (I have a PAL FX1)

No havoc, it is just a difference look with more motion blur. It will not effect 24p prospects.

It will not look film-like.

Not film-like in what way Radek?

Film typically uses a 180 degree shutter which means a 1/48th of second exposure. The motion blur is equivilent to a 360 degree shutter (which is not mechanically possible.) This does not impact you ability to export to film, only the idea that it looks like it was shoot on film -- which is very difficult to do anyway (no a real goal in-itself.)