MJ is about 9kg (im assuming - she has not been weighed in a while)
She is in the medium WOOL covers comfortaby, at a push i can get the small on, but the top hangs out a bit...
She can wear both the small and the medium PUL covers...
And she squeezes into her small AIO - but the rise is a bit of a problem, so we only wear these at home

However, MJ is a long skinny baby (not overly long though)...

So, based on this i would go for the medium for all, you are having another baby, so even if Flynn only wears them a bit, they will be used anyway... and once you decide what you like, then you can order larger sizes when you need...

you may find that you end up using other covers more...

ETA- im on Clayton, so if you want to meet up somewhere you can check out my stash if you like - good idea Mel