@Notsure838 it does all seem very quick, married, moving in together, his kids having been through abuse, however, you appear to have got help from all the right people & the kids sound happy about it all. Sometimes it just right, even when fast.
id go for
2x 4yo
6&9 boys
7&10 girls
With their input on how they want their rooms, bunks/dividers, twin beds, shared separate book cases etc.
ds 6&9 I'd just do what you both think DS6 needs & make it as good for DS9 as you can.
I think it's great to remind them they can talk to you if there are any issues, but be careful not to over promise on being able to change things too much 'we'll see what we can do' rather than 'we'll change the arrangements. Because especially as they get older, there might not be too many other options.
I hope it all works out brilliantly for ALL if you!!
(were you not just a little concerned getting married quite quickly & making it a family with 6 kids?? What age are you two? I think when I was young this would have sounded brilliant, now I'm older (53) it just sounds exhausting 😂😂)