+1 on the MSF class. It’s your life we are talking about here so it’s best to do all you can to stack the odds in your favor.
Can you learn on a Ninja 750? It’s not a simple yes/no answer. The more you move away from the recommended 250 class the more difficult you make things for yourself. Can it be done? Of course it can. Is it recommended? No, probably not.
The other thing I would say is you have to be honest with yourself about your self-control. The one person guaranteed to get hurt if you ride outside your limits is yourself. I’m a 40 something who has never given a crap what other people think (I ride a 3 wheeled scooter for chrissakes). The first time someone gave me a compliment at a light I cracked that throttle open to show what it could do and nearly face planted into a concrete wall. Will you be able to control yourself when riding with your cousin? Will your ego be okay eating his dust? Just saying it’s easy to get suckered into riding outside your limits. If I had been on anything more powerful I would have had a painful crash that night.