About a year ago I bought 2 GSMs from a LFS that had a tank with around 30 of them in it. This is the only time I have seen them grouped like that in a LFS. I bought 2 juveniles, both around the same size. Basically they beat each other up for about a week, then settled down & one grew to around twice the size of the other & they lived as a pair, inseperable, until I broke the tank down to relocate & returned them to the LFS. These guys were VERY territorial, & liked to dig up my sand bed, particularly the bigger one. I had a
bubble algea outbreak & put a foxface in there. That foxface made short work out of the bubble algea everywhere except for the section of the tank occupied by the GSMs, they wouldn't have any of him coming near them, even though the foxface was twice their size, it was the foxface that backed down every time. That's my experience with them, HTH.