1. Are both correct? I'm good at playing soccer. I'm good at soccer.
Both are correct, but "I'm good at soccer" is far better.
"I'm good at playing soccer" sounds a childish. ie. What you'd expect a child up to a certain age to say. An adult would almost always say the 2nd form.
2. Which one sounds more natural? I'm really good at soccer. I'm good at soccer very much. I don't feel like "very much" is used with "be good at," but I might be wrong.
The correct (and natural) form is "I'm really good at soccer".
"I'm good at soccer very much" is incorrect.