The same way you do with chickens. Have them penned where the current flock can see them but not get to them. After while let them all loose together and watch what happens. If the newbies get picked on to much separate them, if they get along fine then they're good to go.
I would wait until at least 2 - 3 weeks. Your going to want them to get some size to them before introducing them. Also keep notice of night time temps if you put them outside before they are feathered. Were I'm at frost is only about 4 weeks from now. I also noticed the lowest member of the chain will be most aggressive with new comers since they will try to show dominance over the new members. There usually isnt any injuries but do watch make sure the newbies get to eat.
Would i be able to get barred rock hens and one roo and have them live with the runners?
Also if i get slower bigger ducks such as pekin or rouen, will the stay in the "herd" when the lighter ducks are on the move?
I believe the best thing to do is decide what you want realistically. Chickens and ducks can live together but the older your flock gets the more difficult it will be to intertwine new members into it and them all take to each other. What will happen with the chickens and ducks will be that the ducks will have their own mini flock and the chickens will have their own mini flock all in the same pen. If you wait to long there is a possibility that new ducks added will make their own mini flock as well and not intertwine with the older ducks. Pekins are not slow ducks they will run and move around like the Runners. Runners are really not that fast, they run like other non-flight ducks do. Or at least the runners , pekins, rounes, ect have that I have seen.
Also make sure you have the space you will need as well as can afford to feed them and have the time for them. Sure this is all fun now when first starting out but ducks can live 20+ years unless you plan to butcher them. They all still need care no matter what the temps are, weather is like, or if you's sick with pneumonia. They dont care if you have work or school. I'm not trying to stop you from getting more I just what to make sure you understand they are not novelty toys to get and over do it in (which is very easy to do) then realize your gotten in to deep. Ducks need more bedding than chickens as well since they sleep and poop in it. Ducks need more floor space than chickens and if penned they need more run space. Ducks need more niacin and protein as well.