I don't know if this would be of any help to you, but I was 2 1/2 when my sister was born and we shared a room right from the beginning. I don't see any problems with a toddler and newborn sharing a room. I presume that the newborn wouldn't be in the room alone with just the toddler all that much at the start, as you would be feeding your newborn every few hours, etc. Having said that, in your situation I would keep the newborn in my room for the first six months or so, not to intentionally keep her away from the little boy, but because I'd want time to be with and bond with my baby. (Hope this makes sense, my brain seems to be kind of slow tonight...)