It is totally dependent on the individual fish.
I have a 9" male (more RD'ish, big lips etc.) and a 7" creamsicle female (more Midas'ish, stout, heavily built), that I put temporarily in a 50gal breeder (36X18).
They were separated by eggcrate that was supported by large suction cups on both sides. After a couple weeks they started knocking the divider down, and I got tired of putting it back up. At first they would lip-lock, and since the male has big lips he didn't ever do any damage to the smaller female. They have been there for about 4 months now (yes, I know, not really temporary lol) and now don't even spar each other. They have spawned a few times and I have not bothered to take any fry out (of course, no room for them!), the fry get consumed after a while.
I await the day when they snap and fight to the death but have no other option right now until I get some time to set up another 75gal so that they may be separated.
If you do anything, just make sure your keep a close eye on the action.
Burt