Introducing kittens to each other / too aggressive?

[prettyVAGUE]

About two weeks ago, I adopted a male kitten, Phoenix, who is about 9 weeks old now. Today, I adopted another kitten, a female named Violet. She is only 6 weeks old. They were both rescues. So, Violet is about half the size of Phoenix... and I'm worried that Phoenix is playing too rough with her. He pounces on her while she's using the litter box, he also pins her down, biting/scratching her to where she squeals. He's continuously following her around trying to get her even when she doesn't seem to want to play. She dishes it out back, even hissed at him when he was trying to get the toy she was playing with. I'm not sure if he's trying to hurt her or if it's okay for them to play while he's making her squeak and not letting up. I've tried putting Phoenix in a separate room overnight but they just cry all night for company. Any help is greatly appreciated.

 

[Chelsea FC]

He's not trying to hurt her, but he could get too rough. Until Violet gets a bit bigger don't leave them alone together. Some one needs to be there to referee.

Kittens do play rough, it's perfectly normal. Just let Violet grow a bit.
Make sure there is somewhere for them each to go to be alone.