I know nothing of sand AND soil mixes, but as long as the mixture is damp and I mean damp enough for the little guys to root, you're fine. Dahlias are fairly bullet-proof. Mist them and if the potting medium becomes a little too dry-dump a little water on them. I mean, most home air is dry, sand drains and I doubt they would rot unless you drenched them.

Congrats on your sprouts. The ones I ordered had no eyes and their little necks are starting to shrivel up.

Well, I was just notified that my big delivery has been shipped! Yikes. I haven't even been able to get into the soil yet!