Minki,
Even if you have got into this situation for all the wrong reasons, you have my sympathy.
Aside from the upheaval of moving two families into one home, you're going to have the added trauma of trying to sort your relationship out with your DP.
To me it does seem like you have netted another dad with kids on the lookout for a stepmum he can dump them on and resume the semi bachelor lifestyle he thought he was going to get when he got divorced but didn't bank on having to look after the kids.
That may sound awful, let's hope it isn't.
But you do have a lot of talking to do before one packing case crosses the threshold. You need to be on the front foot here.
I would start with holding off moving in until after Christmas but invite him and the kids to stay for a few days. I don't think that's unreasonable and someone can sleep on a sofa or can use your office if you're not working.
If you get a bad reaction to that I'd say you're on a hiding to nothing already. Let's pretend you're not.
There are going to be expenses over Christmas - twice as much food and drink of a festive variety, you need to test the water here with a conversation about contributing. Suggest something like 'I'd normally buy a 20 turkey for the three of us, but to feed 6 it'll need to be more like 45 (btw I have no idea how much turkey costs). Then there'll be the extra veg and pudding, xmas crackers, tub of chocs for the xmas day movie etc and one bottle of win isn't going to go very far', give him a few seconds to work out what you're telling him and if he doesn't volunteer just ask him outright. 'How do you want to split that? You buy the turkey, I'll get the rest?'
On the accommodation front, FOR XMAS ONLY, is there any way you can give DSD a room of her own for the few days she's there? At 11 it is not going to be very long before she'll need her privacy. Would one of your kids take up the challenge of sleeping a bit rough, camp bed or blow up mattress and sleeping bag. Maybe you could make the temporary bedroom a bit of a campsite, put up a tent in it??!!
That way you are making a generous offer.
Get that sorted and don't have any more moving in talk until after Christmas day. If you've mamnaged to survive the time together, shared the domestic burdens etc and not split activities by families you can move into Phase 2....
And that would be along the lines of 'I think DSD needs her own room, she's growing up with all that entails, she's not a little girl anymore', if you genuinely cannot work out how you do that follow it up with a 'BUT', and ask if he has any suggestions. Don't get into any arguments about that just talk any ideas through to their logical conclusion/natural death.
On the finances front, yes get the budget sorted. I think a suggestion he contributes much of what he as paying in rent previously towards the mortgage isn't unfair. The work out a split of bills, it should be pretty 50/50 given the numbers we're talking about. When it comes to kids activities and clubs etc, try and avoid going down family lines again. There will be future opportunities and kids give stuff up, don't make it an open wound from the start.
If it helps our arrangements are this
House - not owned 50/50 but furnished and mortgage paid 50/50
Utility Bills - 50/50
Food etc - 50/50
Car bills - 50/50 (except fuel and its whoever uses it pays for it)
DSC's birthday/xmas presents - mostly paid for by DP, I buy one or two things myself (I have my own non resident kids to buy presents for)
Sports/hobbies - tends to be DP but when it comes to random ad-hoc match fees and stuff its whoever's got cash, usually me!
Treats, days out cinema, take aways etc - 50/50 pretty much.
It's probably not perfect but its relatively controversy free. It also gives us the opportunity to offer to help each other out if we're struggling one month - lots of presents, school uniform etc
Good luck. Hope to hear from you at Chistmas having a good time !