I am a stepdad, and rule 1 is very simple indeed -
you will present a united front in front of the children. If you say 'No' to one of 'his'kids (they are all yours now aren't they), then he backs you up, even if he thinks you are wrong, and you discuss it out of earshot of the kids, and they don't know about your disagreement. No "Can I see you in the other room?" with a face like thunder!
This is essential, otherwise your children will play you off against each other - that's human nature, nothing to do with step-parents, all kids do it, it just feeels more sensitive in this situation. You must have a situation where you are saying '"Bob already told you no, so what are you asking me for?"
It's hard at first, but well worth it when you get the rules down so everyone knows where they are.
The additional child issue is something you are going to have to work out - get this part in place first, then move on to that.
If you get resistance, start thinking seriously if co-habbiting is a good idea for all of you.
Good luck.