Do I need pots in a SOG?

I keep reading about people using pots or grow bags in a SOG. Do I need them? I'm using two 4x4 flood tables under three 600w HPS. I am going to root my clones in 1" rockwool then transplant to 4" rockwool in clay pellets directly in the tables. Is that okay. I've spent all my money, and can't even really afford grow bags, much less pots.

Attached is a closeup of one of the trays. Should I put the clay pellets directly into the tray, or should I put down a coco mat first to prevent the pellets from fouling the channels and impeding drainage?

Can anybody help me? Puhleeeze?

If you grow in small containers the plants won't get as big (ideal for SoG). Growing them all in rockwool or hydroton balls with all the roots mixed in one huge tray will give your plants more potential to grow bigger and branch out more (not so good for SoG). Or are you talking about just having one 4" cube per plant and trying to keep the roots from getting tangled up? If you want to dump all the media in one container and grow them all together, maybe try a screen.