New grow questions - transplanting and nutes

Hiyas - running 6 26w cfl's 6500k runing 24/0 - currently have 12 seedlings in partycups in some container mix I got from a big box reatailer - it's not mircale grow.

seedlings are 2-4" inches away from the lights they seem very healthy right now they are on day 18 of breaking soil and they are starting their 3rd or 4th set of true leaves I think. They are only about 2 inches high, there is no stretching going on.

The little round leaves have turned 80% yellow on most of the seeldings - not sure if that is normal.

I am thinking they will need to be transplanted very soon but I don't have any pots so I was thinking of making some out of trash bags or grabage bags maybe some cardboard not sure - my concern is making them strong enough and also I am not sure how to drain them... if I put them into bags and they stand up fine, I am wondering how I will catch the run off? Will I need to contrsuct some kind of run off catcher? Right now I have the party cups on a plastic serving tray and I remove them from the tray and put them all on a towel when I water them and let them sit there for an hour while the towel soaks up ther unoff then I put them back on the tray and back under the lights. With bigger pots/bags there is more runoff.

I am wondering when I should transplant?
Are the little round yellwo leaves an issue?
When shouls I begin nutes? I only have MG 24-8-16 and I'm hesitant to use it cuz it's MG... and how much should I use?
How big of pots should I make?

I plan on vegging them for 60 days total.

Oh how do you guys feel about switching from 24 to 12/12 - should it be done overnight or gradually over 3 days?

Also, I have no mylar, how do you guys feel about using tinfoil to line the grow cab? Does it help or hurt?

thanks

transplant when they are about 2-3 weeks from seed. 4'' or so minimum

and as far as size of pots use the rule of 1 gal per foot of plant. when to use your nutes? does the soil you have them in now have nutes? what dont use tinfoil, tinfoil = fail. white paint, mylar, or even those auto windshield refelectors or emergency blanket

well they ARE 2-3 weeks from seed but they are only 2 inches high... should I transplant now?

they are going to veg for 2 months then flower for 2 months how big should I expect them to get? 4 feet?

There are no nutes in the potting soil that I can see.

bump and any tips on making pots out of bags?

bump... plz help