Black plastic- Get panda film instead and use it white side up.
Exhaust- dryer vents are usually 4"; unless you are running a rather small light like a 400HPS you may want to consider alternate arrangements like a 6" duct with a filter so you can use the heat to heat your house in the winter if you are in a cold weather area. Either way, smell control is a must.
Soil and nutes- you may already have scorched the seedlings with that dirt. Unless it was labelled for seedlings, potting mix from box stores is usually not right. Also MG sucks goat nads and you should head over to a specialty garden center or nursery and poke around to see what else is available, or order pot ferts online if there's no gorw shop nearby. Skimping on dirt and ferts causes headaches for months. Trust me!
Keep us posted and good luck!