Hi and welcome
Please give your girls more time. Guinea pigs are prey animals and not naturally born cuddly toys. The commercial mass breeding of piggies by pet shops and backyard breeders means that they have virtually no human interaction before they are thrust into a complex pet home with expectations they cannot fulfill.
Please provide two huts with two exits each. It is a human misconception that guinea pigs want to snuggle up all the time that is reinforced by all the human-centric videos on social media that cater to what humans want to see and find cute or funny, not to reality or species needs.
Leilei is now starting to be a teenager and is getting confident enough to establish her own group in her new territory. It is also likely that she is coming into her first stronger teenage season.
What you are experiencing is perfectly normal mild dominance behaviour that is at the very core of cavy social life. The rumbling is a mild display of dominance, in sows seen most often during bonding and when they are coming into a stronger season. The submission screaming is the expected reaction to any dominance by the leader and is neither pain or distress. It simply translates as 'I am no rival of yours.'
Please do not separate and allow them to get on socially now that they have settled enough to finally get as far as that. There is not the least need to worry for you. The more you interfere, the more difficult you make it for your piggies. It is hard for new owners to learn to sit by.
Yuki
is sleeping, by the way. Guinea pigs normally sleep with their eyes open and will only sleep with closed eyes if they are very exhausted or feeling very secure in their surroundings; as prey animals they need to be able to come awake in literally the blink of an eye. They also will only sleep outside their denning area once they are feeling safe but there are always some that never will and that prefer their dens.
Please take the time to read these links here to learn more about guinea pig behaviours and where they come from, so you can understand what is happening and will feel hopefully much less stressed.
You should find the guides very helpful. Our 15 years practical forum experience and our own owner experience, which in some cases goes back as far as 50 years have all gone into our step-by-step information and how-to advice guides. Since we cannot repeat all the information in every post, you will often find links with more relevant in-depth information or practical advice in links in our posts.
Settling in and making friends with your piggies:
Arrival in a home from the perspective of pet shop guinea pigs
Understanding Prey Animal Instincts, Guinea Pig Whispering and Cuddling Tips
How To Pick Up And Weigh Your Guinea Pigs Safely (videos)
Understanding behaviour and social interaction:
Sows: Behaviour and female health problems (including ovarian cysts) (includes dominance behaviours in sows and sows in season)
Bonding and Interaction: Illustrated social behaviours and bonding dynamics (with plenty of pictures of key socially interactive behaviours and illustrative videos)
A - Z of Guinea Pig Behaviours
Guinea pigs as a species in their own right:
Guinea Pig Facts - An Overview
Journey through a Lifetime: The Ages of Guinea Pigs (charts the physical and social development from birth until death with some helpful owner aspects)
All these very helpful guides that help you to learn what is normal and what not are part of our New Owners guide collection. You may want to bookmark the link, browse, read and re-read at need as you will take different things from them at different levels of experience. We are of course always here to answer any questions you may have:
Getting Started - Essential Information for New Owners
Reading up is a bit of work for you right now but it is well worth the effort to help you understand better what is going on and to be able to enjoy your piggies more as they really are (a fascinating species and personalities in their own right) and not as you expect them to be.