I am creating a waterfall and I want to have the fluid physics loop over a set of frames I am creating a small waterfall for an animation. I have the ground texture and rocks all created. I am specifically wanting to create an 8 second long animation that will be an endlessly looping video file.

I am well aware of how to work the fluid physics - create the realm where the fluid sim takes place, identifying an object for inflow, labeling the ground and other objects as obstacles, etc.

I found a number of guides and forum posts on blender.stackexchange on how to create a looping ocean, but not a stream.

I am assuming that I would take the fluid physics, duplicate, then scale back to earlier in the keyframes. I would then interpolate the meshes of both fluid physics at a point where its indistinguishable to the viewer. Since the waterfall itself would be the most dynamic aspect, I would assume that I interpolate the mesh before it falls and once it hits so the falling water streams don't show any glaring changes mid-fall.

I am unsure how to change the timing of a physics simulation along the animation strip, and how to interpolate the streams. 

I know that I can add a Cyclic F-modifier to simpler animations, but would something as complex as a stream be able to have that work without a glaring seam?
Your best bet would just be to render out a 10 second sequence (ie. 8 seconds plus a little extra), and then cross fade the clips in a video editor. I don't think you're going to be able to actually loop the fluid sim files with any sort of precision.

You can physically duplicate the fluid sim files that you've cached. If your cache folder contained: fsim1.bobj, fsim2.bobj, fsim3.bobj, then you duplicated them and renamed them fsim1.bob4, fsim5.bobj, fsim6.bobj, then you would then have a 6 frame simulation with the original simulation repeating twice. Figuring out how to blend them would be the difficult part.