I'm happy to report that Newman isn't hissing at Cooper anymore and I'm not seeing any signs of aggression (and Cooper isn't afraid of him). Newman seems to bite him on the back of the neck / scruff area but I imagine it can't be serious since Cooper doesn't yell, or flee...he keeps playing.
My only concern is the 2 of them wrestle and play like kittens while Cooper weighs 3 pounds and Newman is 13. I'm satisfied that Newman doesn't intend to hurt Cooper but the size difference worries me when they wrestle. Is it safe to leave them unattended? They seem to like eachother and Cooper now hates his safe room and Newman walks around chattering and crying when he can't find him.
How much do I need to supervise and worry about the 10 lb. size difference with their playing?
Congrats on Newman accepting your new family member! We had the same worries when Guin first came. I'd watch them to see if Newman gets too rough with the play of if Cooper seems like he's had enough. When one of them is tired of rough-housing they will usually just run off or make it clear it's time to stop. Lancelot still bites Guin on the back of the neck to tell her he's in charge, it's not aggressive but more a heirachy thing. We only get involved if one is obviously distressed. Hope that helps!
It does, thank you! I can't quite decide if I need to return Cooper to his little safe room when I go to work tomorrow. He's been out since Friday night and I think it would be hard on both of them...but Newman gets a bit rough with him a couple of times a day or so during play so I'm not 100% sure how to handle it when no one is home.
I agree with Candy, I did wait though until Dexter was about 5 lbs. before he was left out with "the big kids" when we went out for an evening, I worried most of the time I was gone and came home to all hale and happy, since then he's been fine.
I told Cooper to eat lots so he can grow into a big strong boy who can hold his own faster. He behaves like he weighs 15 pounds...but at 3, he's pretty small!
Glad your boys are now getting along and having fun together! It sounded like a difficult adjustment period...I felt for you.
I'll be getting a second kitten soon, so I am interested to hear the replies to your question. I would be concerned too during this short time when their sizes are so different. Best wishes.

...I just realized, Cooper has tried to nurse on poor Newman!!!
It's definitely a work in progress but overall Newman seems to like Cooper most of the time now.
That is wonderful news ,
I bet you feel so much better that ,,that is over,, that was the hardest part for me with Cello and Leyla,
I was such a wreck. Sorry not much help tho on the playing ,,Cello doesn't play hard the boy doesn't want to get hurt

When Aiden came home he was a skinny little mouser... only 3,3 lbs at 16 weeks - his breeder said he was a mommas boy and prefered to nurse - without that he was a picky eater

Lucky weighs a little over 11 lbs and always has been very boisterous at play and I was a bit concerned at first (Hermes weighs 15,4 lbs but is a very gentle boy). I was home a few days and within that week I noticed Aiden was agile enough not to let Lucky pin him down, so I wasn't too concerned. Now after 2 weeks here he's gained quite a bit of weight (5,5 lbs) and it's fun to watch him already trying to get back at Lucky

So happy that Newman is enjoying Cooper. Snowdrop was always the one chasing Coco to start with, eventually coco realised she was bigger than snowdrop and started to play a bit rough. Personally I didn't like to leave them unsupervised at night or at times when I was out during the day and always separated them, snowdrop was so confident, but still so small. it was about 3 weeks before I left them alone, snowdrop has lots of little hiding places, which are to small for coco, so she has always managed to escape if she needed to.