Personally i'd let your daughter decide. If she's comfortable she'll carry on as usual and it won't bother her (which is probably what will happen) at all. If she feeds through the night i'd keep using all the inserts it came with because its better to be over boosted than under...
Poppy is 7 months too and although she doesn't have any WNNN's yet she does wear NN's with wool over the top and she sleeps just fine. the huge nappies have never bothered her and she's been wearing them since she was about a month old.
Night nappies do take a bit of getting used to because they look so big and its makes you wonder how a baby can be comfy in them but if they work then just stick with it.
Like everyone else has said, shed soon let you know if she wasnt comfortable.
DS cant walk when wearing his NNs (we use UD and WN), he waddles, unable to swing his hips I can't actually see him walk in his nappy at night without going into hysterics, so DH does bedtime alone now
Hope you've managed to try it & it's been successful! We've got a few WNNN & they are huge but so absorbant!! My friends dh was put off by the size when I lent them one to try but was soon converted the next morning when their son's cot was dry!!! He's such a heavy wetter that his sposies were leaking everynight! A WNNN was my dd1's 1st night nappy & she was 20months & she's never complained about the size.