They look not so bad but the key for sog is not that you need a specific sized plant, you just need a massive root system. Even if the plant gorwth is slow and is like 6 inches high and there is a nice root system then it is ready for flower. Also heat/nutes/water and all other stress during flowering period will decrease the yeild dramaticaly.
I'd say, test everythings out while in vegetation so there is no loss in yeild if stress or other sorts of things happen.
You might want to look on RO DI water filter. Plants look much more healthy and water is much more stable after filtration. I had to ajust my ph 3 times a day with the tap water and it tooks lots more ph down than by now, i ajust it everyday or so with few drops.
As for slime, root rot ect.. you might want to add some Hydrogen Peroxide into your rez. I use some 35% but be carefull, you can bleach your skin and kill your plant instantly if not used properly.
You can also add more air to the root zone.
By the pictures i'd say lower the light to 12 inches if possible, and if you had nute burns at 700ppm at 4wks flowering im pretty sure your water is very hard or high in calicium.
Is the system dwc or aero?
- if dwc, do you run any air stones?
- if aero, what is your watering cycle?
Good luck.
