Hi OP, The voice of reason and experience here. Fist things first. Do not ever post 'blended family' questions on AIBU. because if you are a step mum or a potential step mum then you will ALWAYS be wrong. Whatever the question.
I have EXACT experience of your situation. I met my now DH in the December. He moved in in the April. His children started EOI visits at the end of that month. At the time I had a small end of terrace with 2 double and one box room. (Big enough for a single bed)
My kids were 11, 9 and 4. His were 9 7 (autistic- special school) 4 & 3.
As you can see, 9 human beings into 2.5 bedrooms is a squeeze. We found ways. Mattresses on floors / camping in the summer/ Us on a sofa bed.
We found a way. The ONLY important thing in all this is that the kids are happy. The only one who got his own space was my autistic step son who got the box room when he stayed.
It's 4 days a month. We did it for 6 yrs before moving to a bigger house. Only one more bedroom. but eased things a bit especially as they became teenagers. No one died. No one was psychologically damaged for having to compromise their sleeping arrangements 4 days a month !
DH ex went ape shit over the years. Constantly tried to thwart contact. We went to court practically every year for a decade on one nonsense or another . And before anyone starts bleating about 'it's their home too' . It's blatantly NOT. Their home was where THEIR STUFF lives. In a large house with their mother where they each have their own large room.
They VISITED us every other weekend. They were welcomed, loved, and cared for but saying it's someone's 'home' when it blatantly isn't is ridiculous.
As I said, it's about making sure the kids are happy. In our case, so upset by over a decade of floor sleeping/hot bedding and making do - two of his four voted with their feet and moved out of their own comfy home and came to live with us as soon as CAFCASS approved it as their own decision. 12 yrs old & 13 yrs .
Ultimately only you know what's best. We also have 7 together. 3 now at Uni. 5 living with us . 2 visiting EOW.
It isn't his ex decision. As an equal parent he has the right to decide what he does with his children on his time. As long as it idoes not harm their welfare. I know MN thinks that kids not having their own bedroom is tantamount to neglect. I know from experience the courts regard this as total bollocks.