John has answered some of your question as well as they can be answered. It's good to have John on the forums. As far as the line size goes, it is popular to use 5/8" ID weighted hose for the bottom of the pond. The larger size will be easier on your pump. Smaller hose is harder to push air through and will cost you some extra money in the end by higher electrical costs and shorter pump life. If I could change one thing on my system it would be to use the next size up from 1/2" PEX to get from the pump house to the pond. I may be restricting the system a little bit, but not much. I did some bench testing of my system and was goofing around with the 100 foot roll of 1/2" pex and the pressure gage and was surprised how much back pressure the 100 foot roll created when it was hooked up to the pump. I do not remember the pressure readings, but do remember thinking that it was more than I would expect. I went ahead with the 1/2" pex becuase there are three parallel lines after my plumbing tree that triple the cross sectional area and, hence, act like a larger tube. With all that said, the Gast 0523 on has 1/4" NPT holes at the pump which is pretty small compared to the line sizes I used. It's actually the length of the hoses that can create the backpressure issues.
IIRC, it only takes 15 pounds to sink a Vertex single diffuser, so I used 20 - to 25 pounds for good measure and my bases could be wider, but seem to work. They are positioned on the slope of my pond bottom and are a bit "tippy".
