over watering is definitely the most likely culprit. I wouldn't do much squeezing out of the cubes, just don't water them as much. The roots when they first start break pretty easy.

Well, first thing first, you need to just let these babies get big enough to take some clones and find out who the chicks are. Once you have a confirmed lady, it is on. Clone off of her like mad and fill the system. I would say that right now you have about 5 times as many plants as you should have started with.

With 2 600's, you should be able to get 1lb per table so your goal won't be too difficult to hit at all.

I always recommend SCROG because of the hieght control that it offers. So i recommend looking into that when the time comes (which is a ways off)