Hey 
If i was you i wouldn't rehome the male Muscy just yet, but just keep in the back of your mind that it may have to be an option later on. It's hard to say if the Rouens would get on well with just the female Muscys, but there wouldn't be the need for as much competition as with males. The only problems you might have is over nesting places, as female Muscys go VERY broody and will defend their nests, so make sure that if you do put all your ducks together, to allow plenty of areas for resting and any nesting ducks to reduce any arguments.
When i allow my Muscys and Indian Runners together while i'm cleaning out their enclosures they get along generally ok. Theres an occassional arguement but normally the Muscys win and they all get along again. But they are only together for half an hour at the most, so they are never left alone together.
With your ducks i would just continue doing what your doing, keep a good eye on them, if the situation gets worse then keep them apart, and if it gets better then try leaving them together for longer periods, but always be at hand incase anything goes wrong.
Regarding your rabbit, i wouldn't like to say whether it'd be ok or not. Male Muscovys have been known to kill ducklings so it may be abit risky keeping them together.
Hope this has given you some help,
Mel X