The toy issue would be the deal breaker for me.
A baby cannot be in a room with toys appropriate for a 7 year old or a 5 year old and in a few months a 2 turning 3 year old. In child care there is a huge long list of toys not allowed in the 2 yr. old rooms, toddler rooms, and baby rooms. A few are hot wheels cars, barbie shoes, hair barrettes, lego's, polly pockets, puzzles with small pieces, crayons, etc...any item that can go through a toilet paper roll is not allowed in those rooms.
Therefore your argument to your husband is that your new baby can choke to death on the older girls toys and that you are not making them give up all toys that are not baby rattles so they should share the room and have a baby gate that makes the room inaccessible for the little girls. The 2 year old should be out too. She is just too young for those toys too. If they have larger generic toys they can all play together in a family room for family interaction. We have lots of larger cars and baby dolls that can be used by girls of both age groups.