In an MMA match? It wouldn't even be close. Floyd's movement and hugging would be totally nullified by Rousey's ability to take him down, at which point he'd be like a little kid fighting a crocodile. The result of Rousey v Mayweather? Rousey breaks Floyd's arm in under two minutes, wins via Submission, Round 1.
While I agree that Floyd would try to keep Ronda at distance, an mma fight between the two would go much the same way Randy Couture vs James Toney went. Rousey would pursue Mayweather to the cage, hip toss him, and that'd be all she wrote as Ronda broke Floyd's arm. I would absolutely love to watch this scenario play out as it'd be humiliating for Mayweather, but this isn't something any of us is going to see in our lifetimes. Unfortunately. Ha ha.
Well I have to say that it would be really interesting to see. Something about the whole situations makes me think that the powers that be will not let it happen, but I guess you cannot rule anything out at this point. It would sure them both a lot of money, and we all know that they love their money, so who knows. It would probably be a disaster of an event though, and would be all hype and talk and media, with no real event taking place, a lot like the last Mayweather fight.