Back to the OP's question. I think your plan is a good one!
I originally had one of my 10 gallons divided (before i started my sorority) so it was males on the outside and a female in the middle. It worked well, lots of bubble nests at first but they all settled down within a week or two. I think it's a lot less stressful for them to see the opposite sex all day long than the same.
As for moving the mollies, that could work too! I have kept mollies with goldfish before and it was perfectly fine. Just put it at the highest temp the lower end of the warm-water fish's preferances. As an example (not a real example) say a goldfish is 65-74, and a mollies need 70 - 79? Put it right at 72, so theres a little wiggle room for both species with temp fluctuations and they will be just fine.
I would go ahead and do it