I agree you really need to deal with this before you move in together.
However, one thing I will say is that you can have the same rules for all the children but some kids will kick against the boundaries a lot harder than others.
Often it's just down to personality. Some kids are just harder work than others.
My DSS complained that he was treated differently to his sisters. The reality is one one occassion, for example, they were all asked to complete their homework, DSD's just got on with theirs. DSS refused to do his and resulted in a lots of attention and firmness on DP's part. It was the same rule, he just reacted differently to his sisters.
So make sure you are completely clear on what the rules are, but more importantly exactly how you are going to deal with non compliance.
I would slam those brakes on now. Their Dad is obviously not dealing with it sufficiently or they would stop doing it. As Fooso says you need to draw up a list of house rules before moving in and make sure that they will be stuck to. Your dc will resent the sdc if they see them being treated differently and they will also resent your DP as well. It should be one house one set of rules for all or it just won't work. Good luck!
Do you mean living together? If so, you can't really have two sets of rules - one for his kids and rules for yours...your kids will find that very unfair and will cause rows. If you are all moving in together now is a good time to draw up some basic house rules - rules that they can all abide by - set chores, no swearing etc - make it a positive thing so they get rewards if they do it - but definitely you need to address this before you all live together
Soon to be blending full time, spent most weekends and school hols together. All kids will be together 24/7. The step kids are sometimes badly behaved, swear, refuse to do jobs, fight. Not all the time. The dad does deal with it but it still happens again. My kids do not swear, fight or refuse to do jobs. Should i be concerned my kids will adopt this bad behaviour? They have seen it now for years and it hasn't so far, but we've not been in it full time. I'm hoping that my kids have already developed enough to know right from wrong