doesn't matter the size for tank mates, they will destroy any fish. The advantage to getting a small 2 to 3 incher is that you get to watch it grow. However getting an 8 '' usually means your safe , the fish is stronger and will be able to resist sickness and other health problems alot better.
Maybe you can source some other black sand but the only other stuff i ever found is more grey/black. I'd personally use the 2-3mm black gravel that d-pac make. It's light enough for puffers to blow around and heavy enough to not get blown into the filter this is the stuff I mean:
