Agree with @aek91 - spread out the sizes and know that what is marked isn't alway true. 6-9 may mean 5-8 so don't forget you have it and when in doubt try it on them early!
try getting bigger sizes too baby won't use them right away but they'll use them eventually
yes & no, newborn size clothes they out grow faster than say 0-3 months or 3-6 or 6-9
no just get bigger sizes when you do buy stuff cause they grow so fast