Room sharing big age gap

Hi all, I have a sort of dilemma with the rooms in my home. We have a 3 bedrooms house, 2 of them are double (one is a very small double) and 1 single. The single bedroom is downstairs and the other 2 are upstairs. At the moment we sleep in the biggest of the doubles and my stepson in the other double. He is 10 and he stays 4 nights per months and half of the holidays. I will have a baby girl in September and my idea was: the rooms stay as they are until baby is 6 months, as she will sleep with us and I can use the little room downstairs to keep her toys and stuff, for nap time and for me to work from home. Then we swap and we go in the small double and my daughter and stepson will share the big double - at this point my baby’s things will be moved to her bedroom and we can keep using the single room as a studio

My husband would like instead to do the swap now because he would like to prepare something for our girl...as sweet as it sounds, I don’t think it’s a good idea because:

1- in my way I want to give my stepson some time to get used to a baby and see if he is happy to share the room or not- in which case we can organize the single room downstairs to be his own room

2- at the beginning I won’t have much for my daughter...so if we swap now, the room will be more like a whole boy room with toys and stuff of a 10 years old...my idea instead was to wait 6 months so that then we will have a better understanding of what she has and needs

3- the small double would be very cramped if I need to add a crib and baby’s dresser

I have to add that I don’t want this arrangement to prolong over the 12th birthday of my stepson...I think that a 12 yo boy shouldn’t really share the bedroom with a tod sister and he will need much less things too, so in that case we will need to move him to the single room downstairs or just change home...

What you guys think? Do you think we should swap now? Do you think it’s ok for a 10yo to share the room with a baby? Any other suggestions?