Did you end up using sand?
Personally, I've grown my autos in general potting soil(PRO-MIX, Vigoro, Fox Farm), I'm not familiar with raw sand as a medium. If i was you, I would try and attain a large bucket to burry in the ground to isolate the potting soil (w/ holes in the bottom of the bucket).
I would transplant the seedling asap, auto flowers don't seem to like transplanting for some reason, or maybe it's just they don't have time to recover from the small amount of shock because of their short life cycle.
Any "daily" nutrients will do just fine(General Organics, Advanced Nutrients), and for the first 3 or 4 weeks you wont need to feed if you use good potting mix. From then on, I would use a 1/8th of of the suggested dosage on the bottle (so if the bottle suggests 1ml of feed to 1 litre of water, you would use .125ml of feed to a L of water) you'll need some sort of syringe to make accurate feed measurements though.
Use vegetation feed until the upward growth stops, then you're safe to start feeding a small amount of a bloom.