A lot of people have had good success with livebearer tanks that house just males. Pandas are a great choice for smaller tanks. The only thing about if you did away with the cories, you would have no bottom feeders. (like think of tank levels)
I would pick one kind of livebearer or the other. (either guppies or platies)
I personally think that you would be fine with 2-3 pandas. I used to have 3 pandas in my 10 and they were very content. Would not be ideal, but I think that they would be very happy if you provide a structure or cave of some kind that they can chase each other, etc. Most pandas stay small (like less than 1.5"-there are size exceptions depending on the source.)
Just my opinion based on having pandas. I have never had a gourami before.