They are lovely looking fish.

They are wild caught and quite sensitive, so make sure that they are feeding at your lfs prior to purchase.
Although linear water flow is ideal, it is not absolutely essential. Cool water and high oxygen levels are essential. You may want to look at how you are going to keep the pond cool during the summer, and how you are going to deal with surface evaporation (may need a solid lid, and top up losses with RO).
I have a group of 9 in my river tank, and they seem fine together. I got quite a few because SF says they are found in large groups in the wild
http://www.seriouslyfish.com/species/sewellia-lineolata/Although they are said to be relatively aggressive, my gang seem ok as long as each one has a large rock to themselves. They mix well at food time, and mix well with my other fish. When I first got them two of the males had a battle for the best rock, but that was short lived with no harm done (and fascinating to watch).
As for the behaviour of your fish, things might change slightly as your plants grow. However, the rainbow shiners in my river tank are usually low to mid-water, rather than near the top, which seems to be the same as what you are seeing.