T.J.
Hi A.. I have 7 kids and a 4 bedroom house soooo I've been there done that! I would absolutely put the 6 yr old in with the baby for at least a little bit just for your own sanity!! It's much easier to explain to a 6 yr old that she must be quiet and not wake the baby than to explain the same to a 1 yr old! LOL! It will be better IMO to have the younger girls together eventually when both are old enough to sleep together (say ages 2 and 3)and the older daughter is in shcool and needing more "Big-Girl" space.
Good Luck to you and CONGRATULATIONS!!!
T.