The pool above the pond will be an excellent "incubator" as you put it. It appears the water fall between the pool and the pond will prevent the trout from moving up into the pool, however the sculpin and darters will most definitely move into the pond. I am curious as to what species of minnow they are. If you can get a photo or two of them, post them and I'll identify them for you. If I had to guess, I would say they are most likely like stonerollers though. Stone rollers don't prefer cold small streams like that. If I had a guess, I would say 1 species is definitely blacknose daces. They prefer small cold to cool water streams. They need moving water to reproduce so, they won't reproduce in your pond, but they can survive in it. The other species of minnow could be any of a number of species.
If the rainbows don't do so well, the brown trout are a bit more tolerant or higher temps and lower DO's, so they may do better.
Again, congrats on the pond... It is one of my favorites I have seen on here!
