So, I discussed it with my boss, either no great kitchen or a tiler. In short, bathrooms will be tiled by a professional, I'll do the rest.
If it continues like this, I'll do everything on the house myself, I'm the type who does everything himself anyway, and I'd rather be annoyed about my own botch than about that of a professional company that still got money from me.
I still thank you for the lively discussion, it helped me a lot and will surely help others as well.