welcome aboard, sounds like a great way to get your feet wet!

i think a separate system for each pond would be the way to go

as far as "pea gravel" - you don't want something "pea" size, maybe a little bigger, like river gravel, and stay away from limestone (you can test your media with vinegar, if it bubbles you don't want it) - landscaping places are a good source as well as home depot type places

white barrels will break down from uv faster than blue/dark, but the dark colored barrels absorb more heat..i use barrels, and am thinking of some kind of "skirt" around them next summer (seams like a long way off, had a foot of snow yesterday) - the barrels need support across the bottoms and sides, google "travis hughey" for a good barrel stand idea

regarding the lights, i've got 3 t8 pairs across 2 growbeds, and have radish, green peppers and salad mix sprouting, other stuff looks ok, i have 2 pair with "cool" lights, 1 with "warm"

if you have a good ratio of growbeds to pond volume (1:1 is good, 2:1 is better, thats growbed to fishtank) you might want to raise the beds up (so they are easy to sow and harvest) and use sumps if you go with a higher volume of growbed so that you don't drain your ponds when cycling water, or look into "sider valves" and fill growbeds in sequence, with the right filtration, you don't really need to do water changes,, aother way to look at growbed volume is 25lbs of "grown out" fish to 100 gallons of filtration/media, that'not a "law" by any means, but a good rule of thumb.

good luck and take lots of pics