Thanks all for your thoughts and advice on the situation - I know it was a bit soon to be worrying, but I wanted to make sure we were doing the right things.
We're trying to ensure that the 'pack order' is kept - feeding Millie, then Jennie, then Hazel last. greeting them in that order etc..
So a couple of weeks on and Jennie is still a bit grumbly... but we have a play fight situation developing now.. which is nice, but I am worried about how far it will go (there's been a lot of noise at some points ) and of course Jennie shouldn't be charging and jumping about too much so I have to 'break it up' when it gets too lively.
It usually starts with Hazel with her bum in the air, tail wagging, and front legs / chest on the floor barking at Jennie, who stands and wags her tail slowly. Then Jennie will adopt the same position and they bark at one another, until one of them will explode towards the other and then they chase around, 'play' growling and mouthing at one another. Sometimes rolling around on the floor, tumbling about etc.
A couple of times the rolling around on the floor has result in 'pinning' and more aggressive sounding growling / snarling - which I have split up.
Millie watches all of this with disdain.. though she has gotten up and growled at both of them occasionally, and more recently during a bout she's mounted Jennie

which I had to stop cos I was worried about Jennie's back and the extra weight - but I stopped it and fussed Millie to try and re-assert her status as top dog.
Thinking on it, the last time I saw play like this was when either Millie or Jennie were in season and they would play fight.. Hazel isn't in season though (at least there's no blood etc as yet).
It would be useful to get on opinion from one of the behaviourist folk if this play is 'safe' or if its damaging to whatever 'pack set-up' we have. We have always treated Millie as the top dog - she gets fed first etc, although Jennie is the 'possessive' one, Millie normally ignored Jennies possessive behaviour and usually would sit on her if she started it up!

So I'm confused as to who is where, and want to make sure we get it right.
I will try and video the next 'play' session and put a link up for you to watch - in the meantime any thoughts are most welcome

thank you