Thanks.
It would certainly be possible for them to become a fallen empire, and it would be nice to set them up as such, if it wasn't for the fact that their ethos is currently incompatible with their decadent trait.
It indeed depends on whether the Sheelikoi realize the existance of other life outside their Galaxy and the success or failure in reaching for it. The Sheelikean philosophy is such that, when all Galaxy is under their control, not only they'd still be a very small minority of rulers, they'd enforce limitations on their numbers, to keep "balanced" the echosystem.