Hey Hun, I have the BG 3.0 so will answer based on my experience so far

Will they actually fit a newborn?
Yes, i didn't find them too bulky with the NB liner but then I don't know any difference

Can I use a cover on them if they arent snug enough?
I didn't need too

Will I need to purchase some boosters/doublers for night time even tho they are an AIO?
DD wees a lot and from the time she was 2 weeks old we were using the liner that folds over IYKWIM rather than the NB liner for night time - she started sleeping longer through the night when we did this.

Will I need a cover or soaker for night time?
Not sure if these have the water proof outer, if so then no

And lastly should I bother with flushable liners for the first poos?
Nah - just a good rinse is all they need but if you want to get sposie liners then go for it - we got a bulk roll and I refuse to use them NB poo isn't that bad.

Or would it be safe to think they will wash up fine?
Providing you give them a really good rinse they shouldn't stain. FWIW when DH changes pooy naps his have left stains (the sun bleaches these) I am yet to have any stains.

DD was born 3.6 kg and has only ever been in my BG 3.0 I love them and think they are awesome. I do find the larger boosters (not the NB ones) a bit bulky on her but its only at night time and its just a personal preference thing she still moves freely enough etc.

Best of luck hun

Nae x