When I moved in with DP (and therefore acquired a part time DSS) one tricky thing was that I didn't necessarily have reasonable expectations of childrens' behaviour. I didn't have children of my own, nor did my friends, nor had I been around children much since I was one! I slowly realised I was judging DSS too much by adult standards of behaviour and manners.
I wasn't the type to get cross and try and discipline DSS (plus he was only here eow), but I would spend quite a bit of time inwardly seething!
Over time, and with a lot of help from reading mumsnet stuff, I developed more of an idea of what could and couldn't be reasonably expected of a child of his age. Then I started to pick my battles, decide which things were important to me and which I could live with. For example: couldnt live with - him never flushing the toilet, him never ever owning up to doing anything wrong or breaking anything even when it was obvious it was him! These things I brought up with DP, and also gently started to try and address myself. Other things - like leaving his shoes in the middle of the floor, or bouncing all over the sofa or making a mess everywhere - I let go and just accepted and learned to live with.
I also found that DP was more receptive to me bringing up things that I wanted addressed if I also took the time to point out the great things about DSS and praise him when he was being good. Naturally, when I was in a phase where I tended to be more grumpy about DSS than positive, he would get defensive and we would both end up cross with each other. Now, he knows I do care about and appreciate DSS, so when I bring something up he is much more receptive and supportive.
As I read this back, I realise it is advice for your DP and not for you! But maybe this could be of some use to you in understanding some aspects of how he might be feeling, if he was anything like me. Plus I imagine the fact that you are all moving into HIS home, and presumably the children will be there pretty much all the time, could make it a lot more intense. You'll both have to compromise, find house rules and expectations that you can both reasonably enforce while also agreeing on what things you can let slide.