Will they actually fit a newborn?
My one size fits all don't fit my newborn- far too bulky!

Can I use a cover on them if they arent snug enough?
You can but will only add to the bulk and newborn clothing doesn't accommodate for cloth nappies as it is!

Will I need to purchase some boosters/doublers for night time even tho they are an AIO?
Not for a newborn I wouldn't think. They don't wee that much!

Will I need a cover or soaker for night time?
You will change thru the night anyway so not for a newborn.

And lastly should I bother with flushable liners for the first poos?
For the first few days I would use disposables but after that you will be fine.

Or would it be safe to think they will wash up fine?
Breastfeeding poos stain in the wash but then the stains disappear once they have been on the line in the sun.

I have MCNs for my DS but still have him in disposables due to the size of the MCNs and he is a big baby!