Before you think you want to expand, you should have a few crops under your belt with your waterfarm.
Your water level in your buckets should come right up to the bottom of the try that your hydroton sits in. I would also keep it running 24/7 as long as they are in hydroton.
I am assuming you have a PPM/EC meter. IF you don't RUN to the nearest store and get one. You should be around 1000 -1200 PPM at this stage. Check the 2 plants that are not doing well if they have rockwool in them, because then that may well be the problem they are not doing well. Rockwool holds water so much that it will drown the roots in a system like yours.

You don't have to bring in additional fresh air from the outside if its hotter than what it is in your A/C'd room. You will be short on CO2 in there, but that is not much worry about now, but may be in the future.

I think the waterfarm system is an excellent system for anyone to get their feet wet in. Don't even consider going to another system until you have this one down pat.

Spaceneedle