Daisygirl - I love your profile pic! A baby Kiwi! I wonder if we can even get them here in the US? I wonder if they are hard to raise?
In regards to intergrating the two flocks, yes, you should keep them seperated just while they acculmate to the new enviroment and develop immunities to any new "germs". Most people do this for two-three weeks.
When you finally put them all together, I would recommend doing it at night....this step seems to make the transition easier, perhaps because the chickens are exposed to eachother by sound and smell rather than visually (to start out with). Make sure there are additional waters/feeders, in case any are bullied and kept from getting to the feed/water. Check on them early and regularly so you can catch any fights or injuries quick enough to prevent anything serious from happening.
'Hope that helps,
--PC