Hi sorry got back to you so late just noticed your post today.
With any type of lettuce I always start them off in planter pots with very moist soil. A couple
seeds in each they will sprout very quickly. Once they are say 1 inch tall you can transfer to rockwool cubes. I usually split the rockwool cubes on 1 side up to the center. I then take the small lettuce plant place the root down the center of the cube, close the cube, place cube in net pot, fill sides with hydroton then put in cloner. Keep in cloner until you see roots starting to show then place in the area you want to let grow till finish.
10' X 6" PVC are expensive. Are you having problems with clogging sprayers? If yes you should put screens at the pipes leading to the main pipe that empties into the reservoir. You should also place a screen right where the water enters the reservoir.
I have all my tomato plants in the cloner waiting to be put in the buckets either by the end of this week or next.