Hi, I had someone draw this up for me for my system. I'm going to have 12 loops, with 2 6 loop manifolds in series. Please see drawing below. Am I missing anything? I was thinking I was missing expansion tank and worried about low temps returning to existing cast iron boiler. What do you guys think?

I think you missed the attachment.

Possible things you might need, depending on what is on the boiler side -
Expansion tank
Air Separator
Fill and drain valves
Temp and pressure gauges
Pressure relief valve
Isolation Flanges on the pump(s)

The mixing valve will not necessarily protect a low efficiency cast iron boiler. Much depends on the load. What is the manifold hooked up to?