You should expect some fighting between the clownfish. That will determine who is the dominant of the two and that will eventually become the female.
My guess is the size difference between the two and the aggressiveness of the clownfish type will determine how long the fighting goes on.
I have a mated pair of cinnamon clowns who have been together for about 5 months and a pair of onyx clowns which I have just put together. The difference between the two types is like night and day.
My cinnamon clowns, which are known to be aggressive, fought like crazy for about a week with the smaller of the two getting beat up pretty bad. It got to the point where I was about ready to give up and take him out, but I guess he fell into line and now everything is good.
With my onyx clowns, there has been very little squabbling. The larger one will chase the smaller one sort of half-heartedly and then come and swim by him, so it looks like things will be a lot easier with them.
The only clowns that I have heard are the exception are the white-striped maroons. A buddy of mine had a 5 year old female that killed two smaller clowns before he gave up. I think that the trick is to get them when they are relatively young.