Wont be able to tell sex for a bit...just look for small white hairs or clusters of balls. This is located by the branching nodes normally. You can be 100% till 12/12 though, but you can be 95%. Depending on the strain I would say your plants could take around 800ppm (so if your water is 200ppm its 1000ppm) then bump it up as needed. You should be feeding more N than P&K. I use 15ml Grow 10ml Micro 5ml Bloom per gallon for veg growth. You plants will let you know if they need something more.

Your res is way too small for 2 plants, get a 18 gal rubbermaid and put 2 in there. Its much easier with a larger res. Less PH fluctuation and change it much less. You need 5 gal per plant at least. The roots will be mostly in the water...I leave about 3" of space between bottom of the pots and the water level. The key is dissolved O2...you need to keep the water temps 58-70 that is key and a good airpump with a stone in the reservoir.

Exposing the roots to light for a few min is no big deal. When I change my res out I have another 18 gal res thats filled with phed water. I take the lid with the plants off and put it on the other container so the roots dont dry. Then I empty my res out and pump the phed water (the one the plants are in) into the main res. I then Mix nutes and re PH as needed then I pop the plants back in. Try to minimize any time out of the water for the roots.

H2O2 is also a good preventive measure for DWC, its no good for organic but great for what you are using ( I use GH 3 part ). I add 15 ml per gallon every once and a while, this provides more O2 for the roots and prevents algae growth...the main part to preventing algae growth is the res temps and blocking light from entering the res.

Hope this helps!!! Keep us updated.