Your seeds need to be started in grow cubes which are usually made from rock wool or peat which will not hurt your system.

They should be transplanted to your system when they have two real leaves showing and not before.

Lettuce does real well in a raft system. Check out this video...I realize it is a hydroponic system but that has no bearing on the fact the lettuce is doing fantastic in a raft syste.

Is it Lettuce or $$$$$$

Additionally, the lighting should be as close as possible to your plants as long as it does not burn the leaves and should be raised periodically to account for growth.