Yes, their rooms will stay cleaner when there's one play room and one bedroom. You just close the door to the playroom so you don't see all the mess, and their bedroom is tidy. at least, that's my experience.
My two oldest boys share a room since we moved 5 months ago. They are 2 years apart, 4,5 and 2,5. They love it to share the room. They do not wake up if one of them starts crying at night. But at bed time, yes, that's pary time! The first weeks it drove me crazy, everybody told me it would only take a few days to get them used to it. But they kept on partying. So we thought of a solution, and came up with a cooking timer alarm. The oldest sets the alarm (he chooses a number between 5 and 10 minutes) the youngest climbes in his bunkbed, and they can party till the alarm rings. After that, they have to return to their own beds, and have to be quiet. For us it works. (for 4 months now, and I'm still surprised hi hi hi)
I have no opinion about the mixed gender thing, but I don't think that's a problem.
Good luck, and let us know in a couple of weeks!
I.