If I had a thriving chaeto sump refugium would it be able to feed pipefish for their entire lives as long as the sump received enough food? If so what would the scale look like? How large of a sump/tank/ how much liverock etc...
Depends on the pipefish species/number of pipefish. There are enough pods in my 125g tank to feed at least 2 spawning bluestripe pipefish (they're trained onto frozen now, but they were thriving on the pod population alone for a few months), but I have no other major pod-eaters. The tank only has around 75 pounds of live rock, but it's got lots of seagrass and macroalgae for pods to live in. For some reason there are very few pods in the refugium, so the display tank provides them. If you add lots of phytoplankton or spirulina powder frequently, the copepod population will explode.
I can't answer your sump size question, but the bigger the better.
It's easy to get pipefish trained onto frozen, though. What worked for me and many others was feeding the tank frozen food (Hikari mysis or some kind of frozen copepods) and just waiting. It only took a week or two of feeding mysis until the pipefish started eating them.