Litter buddies play fight all the time. It's a learning experience for them. Yells are normal as it's basically them figuring out the limits between play and painful. They should be respectful of each other and be gentler if one yells. Sometimes one just doesn't want to play and the other does, causing the former to over react when 'attacked', so don't always think it's aggressive.
Play fighting is healthy and helps them develop required cat skills, from how to use their bodies to how to read body language.
I've had my cats over 3 years now and they still play fight. Watch the body language, if one is rigid and shrinking their size whilst trying to back away, they're not enjoying it. If an agressor goes wide eyed and ears pin back, they're also in hunting mode not play mode. As long as they're not cutting each other and ripping fur out, it's just play. Oh hissing/growling/howling are all bad signs too, as they're territorial cat noises. Last thing you want is cats who live together getting territorial.
The usual oi and gentle nose flick when they push it too far can help. Now I just have to stand up when one of my cats is getting aggressive, as they know I'm coming to yell at them lol.
They will bite, claw rabbit kick each other, wrestle, flip over and run/hide/chase when playing. Their claws should be retracted and the bites never deep when playing but I understand why it may look bad .
Good shout on deeper litter btw. They like to dig and cover their waste. Hiding the smell is their top priority.
Cat trees are hit and miss. Some cats love them. Others just sleep on them, and some just ignore them sadly. Cats like to climb though so hopefully they'll use it.
Thanks for the breakdown Matty! Kittens play fighting is something I was prepared for, but I've just been surprised by how fast and aggressive they are.. I don't see any fur flying yet but I hear lots of squeals/yowls.. no hissing or growling though... Unfortunately the discouragement technique of exaggerated noises and louder voice hasn't been effective yet.. I just spent ten minutes trying to get them to stop biting my toes and ended up having to go into the office and close the door.