I just dumped out three cups of soil, after two attempts to get seeds to sprout in them. They each had two seeds planted into them. The first ones didn't sprout so I just stuck another one in the same cup and tried again. Nope, after digging around, no sign of any of the six seeds.
They'd been soaked and cracked before planting.
Doc has repeatedly said that he doesn't know of any issues with sprouting in Kit soil, but many of us have had seeds disappear. I never had any issues starting in straight, bland, Pro-Mix, so I may return to that.
Coco? Dunno. I suppose as long as it's bland and pH neutral, it should be fine. We're only sprouting after all. As long as they go right into bigger containers with the correct soil as soon as they grow enough roots, it shouldn't matter.