12 gauge is what I'd personally use, if you used 16 gauge I would consider that as an electrical hazard as there would be far too much current for that size of wire. So to answer your question, yes that would definitely cause electrical issues.
Assuming the breaker terminal screw is tight, an easy way to find out would be to use a non-contact voltage detector right before, and after on wire corresponding to that breaker with the panel cover off. This will easily identify a faulty breaker. without even needing to touch anything...
Would it be a bad idea to start two from seed to finish and how does my plan of main-lining sound as well, would this be an efficient method given my setup?
As we all know, it's nearly impossible to predict yield without a library of information. I've done extensive research and am ready to ask the question. What would be the most efficient way to design this box for maximum yield? I am looking to grow mainly indica plants such as Northern Lights...