It sounds more like a defense mechanism than true aggression to me. If I tried to roll Cody on his back, very gently, to check his tummy and groin area for bumps, anything, he used to respond by struggling like crazy to get away and then growling or snapping, if I didn't stop, which I couldn't. I started it and had to slowly get myself out of the situation, without letting him win. Don't ever put yourself in a position of being forced to let him win something. BUT, when he reacted like this, his eyes were rolling, showing the whites, and he was just trying to get away. He had a truly horrible life, prior to rescue. I learned, if I needed to check his groin or tummy, wait until he was laying on his side (which he didn't do for the first 6 - 8 months we had him), stroke him, talk to him and lift his legs to see the area I needed to see. Some dogs don't respond well to being rolled and I would never do it with mine. In play, they will roll around, all but Cody, but I would never roll them. I can lay on them, lean over their shoulders and reach down and hug/hold them. For you to be dominant, does not mean you must be able to physically dominate them. I could not physically dominate any of mine. Jake is the lightest at 100lbs., only 5 lbs. less than I weigh, but he is much, much stronger. They obey and respect me because I have earned it. I provide all the wonderful things in their lives and they know it. NILF sounds like a good way to establish that. I don't use it, but a lot of people have great success with it. I would not be concerned about aggression in a puppy, until I had worked with that dog and given them a chance to learn my rules and my way. Just relax and start trying other things. I think you will be pleasantly surprised. Good luck
Kathy