New kitten, resident cat, rough play?

I recently introduced a new 8 week female kitten called Chicky to my 5 year old indoor cat Remix, a male who has been desexed. When introduced everything was fine apart from the expected arched poses and occasional hiss but that died down within a few days of supervised meetings. Now my problem is that, every time I put them together Remix will stalk Chicky with pupils the size of dinner plates, swat at her a little and dive on her and bite at her neck/belly. Most people are saying that this is a mix of play and Remix showing who is boss but sometimes she squeaks and I can't help but intervene. Is this kind of behaviour ok? Should I be intervening or should I let them get on with it?

I was also hoping someone could tell me what the best age to have Chicky desexed is ?

Chicky is young and unable to take the rough housing by Remix which is very normal. But she will get bigger in no time. I would let them be but interfere when it gets too much. 

Best time for desexing? I bring mine when they're almost 6 months because the vet will not do it earlier. But I've read that cats can be spayed/neutered at 8 weeks depending also on their weight.