Too rough play fighting

I separated my girls yesterday as I felt that my young one, Sneki's play fighting became too rough.

She's bolder and bolder every day and yesterday I heard Dumbika, the older girl, is making some short peeps, while Sneki was turning her on the back, bite her neck and ran away.

I checked Dumbika and she had some scratches, some fur missing, but no blood.

I separated them just to be safe while I was at work and they were immediately trying to reach eachother.

I don't think Sneki is aggressive whatsoever, she just doesn't know when to stop, and Dumbika is too sweet to teach her a lesson.

I feel bad having them separated as they were snuggling, sleeping together peacefully and so on.

What should I do next? Please, help. I don't wan't to make things worse, but also I don't wan't to keep them separated.

From the description, it sounded like a dominance grooming. The more dominant rat flips the submissive rat onto their side or back and then forcefully grooms their shoulders, front legs, neck, and face. The submissive rat will often squeak or make peeping noises. I've noticed some rats "enjoy" this because their head and shoulders are a very hard place for them to groom.

Young rats are VERY playful and sometimes get carried away. They're learning what does and doesn't hurt other rats. Just like puppies, the babies are learning how to treat each other and how to interact safely. I also think baby rats are just more likely to be vocal about anything since they're young. I have three baby males right now and they peep and squeak, moreso than the adults of my cage.

As long as there's no blood, I'd put them back together.

It sounds like normal baby rat behaviour

please see examples of rat behaviour, including playing at joinrats.com

I've put them together in the play area and observing them.

Dumbika is following Sneki around while sniffing her rear, Sneki's ignoring her for a while, then out of the sudden Sneki flips Dumbika on her back and targeting her neck and legs. Soon Sneki runs away in a happy gallop.

They might started to establish their hierarchy.

There were no scratches, missing fur or anything else violent this time. Right now they're sleeping together as always.
I'll keep an eye on them, but hopefully Sneki did learn how to treat Dumbika without hurting her.

It's interesting that Sneki is also playing with my hand a lot, although she has never been too rough to me. She was always very gentle.

Hopefully I'll get another young girl for Sneki soon. She's apparently waiting for me, but the girl I'll get her from is very busy, so I'll have to wait for some more time.

Edit: I did watch those videos. However, it's hard for me to watch one of my girls being in pain. I was thinking: better safe than sorry. I hope I didn't do any damage by separating them.

Are you sure she was in pain and not yelling because she didn't like it

I'm not sure, but missing fur and scratches...
That has to hurt.

As said earlier, baby rats sometimes get carried away or aren't aware of how their nails and teeth may affect others. They're just a little too rough sometimes. I knew of someone's baby rat who got his leg caught in something and ended up spraining it (vet said the joints and bones looked fine, no damage, just a sprain). Like humans, baby rats have to build up life experiences and learn social cues among each other.
They'll calm down with time.

Rats are pretty smart and know we aren't like them. We have no fur or really extra skin to protect us from their teeth and nails. All of my rats play rough with each other, but never roughly with my hand.