1.) The AP 100 is rated at 5.29 CFM airflow whereas the diffuser maxes out around 4.2. This is concerning to me, but without a pump curve for the pump, it is impossible to say how close you are exceeding the diffuser rating. I can't quickly find a pump curve on the net. If the diffuser is at the 6' depth and has 80' of 3/4" hose...you will see about 3.25 psi of head pressure. But, again, without a pump curve to tell us what the output will be at this pressure...your guess is as good a mine. I'd say you are close to the max rating as is.
The AP-100 would run two diffusers rated from 1.4 – 4.2cfm safely. I recommend adding an additional 9" to this line to extend their life and give you another point of aeration as you mentioned.
The AP 60 (rated at a max 3.18 cfm) could run one all by itself and be in a good efficiency range. Running two off of an AP-60 will not yield much, if any, more water circulation, but will allow you to put that circulation where you want it. This concept works well when surface conditions are as important as water circulation. You are basically cutting the water moving power of one diffuser and splitting it between the two, but putting that split power where you want it.
I have chosen to have more diffusers in my pond than required for a full turn over per day to aid in surface conditions. I get little wind action and don't like floaties. With your FA mats, I suggest putting 3 diffusers in fairly close to the center (or center line) of your pond to push the mats out to the edges AND rake the FA out. If you are not satisfied with the pushing power of the diffuser's boils, you can raise them in the water column and get more action out of them with a reduction of water turn over. Off the cuff, I would say that you would have plenty of water movement if your diffusers were only 3- 4 feet deep.
The key here is to remove the FA and anything (the muck) that is feeding it.
