I vote for basic house rules. Everybody eats at the table. No phone calls after 10PM. No friends over when you are not home. Etc. Whatever you decide to be the foundation of this household.
I think that the big discipline comes from you for your kids and he for his. I have never grounded my stepkids. DH does that. But I do enforce house rules. If the rule is that Johnny needs to clear the table, I'll say, "Johnny, please clear the table. You can watch your show after it's done."
And just like with any home, sometimes how Mom or Dad or Stepmom or Stepdad does it isn't all the same, but it gets the job done. DH might tell Johnny to clear the table at the commercial. As long as it IS done (and the cat hasn't helped herself), I don't comment.
Remember that all of these relationships evolve over time. Start off slow. Back each other up. Don't expect everyone to love everyone, but if you can all show respect then that's a very good start.
With the ages of the kids, why not sit down and have a meeting? Let them give input. What works? What doesn't? What is vetoed by the adults and why? (Before this, meet with BF and go over some non-negotiables so you are on the same page).
I think one of the biggest mistakes my mom made was to not lay out the rules, change rules on a whim or allow our then-stepfather to change rules that made no sense. He had no idea and was just trying to show off. We were 14 and 18 and he put a lock on the cable channels - to show he could. It made for a really horrible adolescence for my sister. Our mom gave him too much control and didn't stand up to him when he was wrong. Don't go so far the other way that you lose your say on your own kids.
There may also be places where you really don't agree. Your 14 yr old doesn't have a phone but his does - it's a place where you may need to explain that it's out of your hands to your child. In my case, their mom bought the phones so SD had one at 10 (and broke it, but that's another story). MY DD will not have a phone at 10. But maybe you and BF agree that his kid keeps the phone, but has to hand it over at bedtime. That sort of thing.