The better effluent zone would be about mid-way up the volume of the tank. Culhane has been working on and slowly modifying this system for a long time and is a professor at Mercy College(?) (Mercy something or other) and has had one, until recently, at home, for years. He's THE go-to on this style. In put tends to do 2 things. Sink, or float. Heavy stuff will tend to digest and rise, while floating stuff tends to create scum, just like a septic tank.
You can stir with all of the gas, unless you have a specific method for separating the CO2. My system just tends to capture most of the CO2 in one place, since it is a multiple-chamber design.
If I can get mine finished, I have access to Llama and Alpaca poop, which is supposed to have more efficient critters in it. Llamas and Alpacas can do well on low-quality feed, so their microbes have to be hardier (that's the theory, anyway).
People have actually started digesters with everything from baby diaper poop, to ground-up fish guts, mud from near-frozen lakes, and so on. Just need a source you trust and enough material to work with.