Is this type of movement achievable in Cascade?
There are a few ways you could achieve this type of movement which I can think of.
To do the fish animation (the body movements etc) I would recommend using a vertex shader on a mesh emitter, to do the randomization of the movement (the fish moving independently as part of a group) I would try using an attractor which moves around inside the group (you can animate attractor position with a constant curve) and set the attraction values fairly low. To move the entire school of fish you would need to put them into local space, and then animate their position in Matinee or a Blueprint.
Another way to control this without Matinee/animation would be to use an attractor and set InstanceParameters to control all of the attraction settings, then move the attractor around in world space using some Blueprint Logic.
Looks like Eric answered this in a separate thread?