MEP's sub adults play breeding

[HouKen]

I have been raising MEP's for 8 years or so. My first 2 were male and began humping each other from the time they got to be about 8-10 inches. I bought 2 that were sold as female. And tonight, I saw that one of the females humping the smaller females. I know it's difficult to sex accurately at that size but, has anyone ever seen girls humping smaller in MEP's? I've never seen it in any of my other species. But, I'm afraid that one of the girls just may be a boy... oh, and when I went and removed it, I didn't see a spaceship.