Never managed to find what was causing issues, so I started a new pack from scratch. Much smaller and centered around GT. All is working fine now. World gen is amazing. (Using PFAA World Gen with GT Ores.)
Now for the main question. I think I've seen somewhere that GT is incompatible with AppleCore... Is that true ?
EDIT : By the by, my server will be open soon enough. Since it's centered around GT (and RotaryCraft.) think I could post about it somewhere in here ? And that people would be interested ?