The size of your sand bed is at about the minimum size that I've been able to keep viable long term. Any smaller and it quickly loses its diversity of critters and starts to collect detritus. Apparently, from what I've read, there are critters that avoid walls & rock and will only live in the open sandbed, there are critters that only live near walls & rock, and there are critters that are happy anywhere. It seems that small sandbeds are typically devoid of the first type.
I'm thinking that if you place rocks in the center you'll eliminate the open sand bed area and then quickly lose a lot of critter diversity.
If you wake up breathing CONGRATULATIONS!!! You get another chance to do it right.