Couple of points.
1) letting the water flow out to transfer fish to the bigger pond in the flow won't guarantee that a predator won't swim in.......
2) +2 on the Pentair micropore diffusers BUT get the Pointfour ones NOT the generic Pentair ones. The Pentair ones will crack over time, either the stone cracks or the junction of the plastic perimeter to the ceramic stone cracks. Do NOT run O2 thru them when they are out of the water then put them in the water, you will be buying new ones.....
3) You want your seine at least 1/4 taller than the water you are seining, AND you want the mesh smaller than the smallest fish (vertical height) in the pond or your seine will act as a gill net. Those small fish that are gilled are a royal PITA to get out of the net.
4) When you seine your pond it will be a LOT easier on the fish if you seine 1/3 then 1/2, then 3/4 then the whole pond. That way you won't have a BUNCH of fish in the seine muddying up the water and stressing them out. Less fish to move too, so it's less stressful on you.

Just plan the pond sides with a gentle enough slope so they don't slough off over time.