There is no such thing that if if you do over shoulder exercise your height won't increase. THAT IS NOT A FACT. Become more knowledgeable everyday about everything and spread it. My advice is  you can not only do dumbells but you should be doing every workout possible. Set a goal for gym, and run for it you can achieve anything.

Yes under proper supervision

Yes you can. It will not affect your height growth. In fact if you do stretching you can increase your height.

Yes you can join gym but do moderate workouts only and more importantly don’t take steroids for faster weight gain . I REPEAT DONT TAKE STEROIDS

Do you really need dumbbell for maintaining body?

Nothing like that but have to be done under guidance bcz vigorous workout has its own complications like dehydration kidney injury etc

Yes you can join gym... But do excercises according to the trainer advice