welcome to the wonderful wet word of hydro.

1- Since you will be changing your water every 2 weeks or sooner the 14 gallon ones just take more water and nutes to run. I think a waste. 10 gallon for 2 plants is fine.

2- Yes cut the hole slightly smaller than the netpot. So it doesn't fall through.
Netpots never go under water. Water level should be just under the netpot with bubbles from the airstone splashing on it.

3- That's a good question. I have no idea.
Really it is strain dependant and how well you treat them and light. How long you veg and how big they get will be the end factor.
In a ten gallon tub, drip system with 6 netpots 3 inches each we have gotten over 12 oz's. 1 month for veg 2 months flower. Under a 1000 HPS.

4- About 4 ten gallon tubs. Maybe 6 under the 600 HPS.

5- latewoods legacy with General Hydroponics 3 part nutes. There is a sticky thread in the hydro section. All beginners should try that first. With some cal mag plus if your water needs it. If you don't quite get it just ask we will break it down. No since in all the add ons untill you get a few grows under your belt.

You can get by with out a TDS/EC meter BUT and this is a big BUT.

You need a PH meter. It is vital to good health of your plants. When PH is kept with in the limits you need for hydro most everything else comes easy. Kind of. PH 5.2 to 5.8.

Hope this helps and keep reading and asking questions.