you can grow seedlings under hid lighting but it needs to be atleast 1-1.5' away from the tops of seedling.

on ventilation look at diy airvents, or you can use pvc piping. you dont want any light leaks period.

pot size depends on how long you veg for or how long they grow for. you want about 1 gallon of soil per foot of plant

mylar is expensive and gets dirty fairly quickly. flat white paint is great and cheap to work with

on the floor of your closet i would put some heavy plastic(painters plastic)

temps should stay around 70-80 is optimal

good luck and read