OK, here's a more detailed explanation of my experiences keeping O. scyllarus shimp in a 6g Eclipse.
My first shrimp, Grendel, died shortly after his first molt. He had been quite active and hungry for the months I had kept him before his molt. I believe that either: 1) the fumes from my wife's nail polish / nail polish remover did him in after his stressful molt (not highly likely); or 2) I missed a piece of his shed exoskelton and/or uneaten food, which caused a surge in nitrates and killed him off. Grendel was a tough loss -- I swear that he was more intelligent than some people I know. BTW, I believe Grendel was a "he" because of his striking emerald color.
My second shrimp, Gorlak, was a female O. scyllarus (I believe so because of her olive color -- I'm probably wrong on this). Gorlak was missing one of her raptorial appendages when I rescued her from a really cruddy LFS. Gorlak died shortly after her first molt (as in, I left for the day knowing that a molt was imminent and came back to find a molted exoskelton and a dead shrimp). It was interesting to note that her freshly-molted body was more vibrantly colored than before, and that she had regenerated her missing raptorial appendage. I believe that Gorlak died from a nitrate surge caused by her shed exoskeleton.
The moral of this story is . . . my two shrimps did GREAT in the 6g eclipse setups, until the stress of molting combined with increased nitrates did them in.
I won't try another O. scyllarus in a small tank. They're beautiful and intelligent animals, but are quite sensitive to nitrate buildup. I think it's possible to successfully keep an O. scyllarus in a small tank for a long time, if one is really diligent about tank care(feeding sparsely, frequent 1g water changes, and being EXCEEDINGLY careful to avoid stressing the bug before, during, and after a molt).
I'm currently keeping a G. platysoma in a 6g. I've also filled the 6g with macroalgae, to help with nutrient export. I feed very lightly (1-2x week), and am going to be very careful when it becomes time for the G. platysoma to molt. Wish me luck.