Sorry for not answering your questions sooner but I got five things going at once. Got to make a
living.
For a 10 x 20 greenhouse I would space them 24 inches from center to center. I would add as many
rows as you can with room to walk between rows for harvesting. Tomato plants can get big so I
would give each plant 4 sq. ft. for room to grow.
Yes I drip feed from the top 24 hours but you don't need to when the sun goes down. A photocell
switch would work well. With hydroponics you want to automate as much as possible.
Gravel is ok if it is porous enough. I prefer
grow rocks because it is lite weight. Gravel can get pretty
heavy.
Here in Florida we have to use swamp coolers to cool greenhouses even in the winter.
Ron