Actually I have 9 plants under the 600W the containers are 3.5Gabout 18 ltrs isnt 600w abit much for a 600w with 2 plants 250watts is more than enough to grow or am i mistakendepending on the size of your grow area aswell, i take it just these 2 plants are going under your 600w if so I wud personally put them in bigger pots and veg them another week under the 600w to get anything decent weight wise.
I use canna nutes (growing hydro at moment) i cant compare because i've never used any other brand, check out my sig im doing 2 under a 600w they were similar size to yours when i got them.
The main stems are quite fat i have the fan on all the time maybe as thick as 2 cigaretsI think newbies are actually better-off doing a straight organic grow unless they are doing hydro. Using chemicals makes the process much more difficult, since your EC, ph etc all need to be watched fairly closely.
Worm compost is easy to get hold of, easy to use, and will not kill your plants if you use too much. It may mean you get slightly smaller yields than if you pump the plant full of chemicals, but not by much if you're doing it right.
How thick are the main stems? They look pretty slim, which is not a great sign yield-wise. Your plant is unlikely to produce more weight than the main stem can hold...