I am wondering if I where to fix just the 2 middle screws as seen in the pic to the stud framing and the other 4 outside screws to just the plasterboard will the TV weighing at 15kg and the bracket stay up? Or will I have to buy a long bracket so I fix all 4 screws to the stud?
six fixings carrying a total of 15kg will give two and a half kg per fixing. this should be fine, especially if two of those fixings are securely in studs.