UQ Planning Questions

[Rabid Deer]

Well, I've got my WBBB and SF, which I haven't even had time to set up yet, buy I'm already planning the next step. It's underquilt time. I'm planning on making a winter UQ first, as that is the type of camping I will be doing most. I live in MN and I've camped on the ground in -20* so I'm planning to do the same in a hammock.
Here is my sleeping bag that I will be using as a topquilt:
http://www.kelty.com/p-467-mistral-2...ping-bags-pads
My thought was to use this bag as an UQ:
http://www.kelty.com/p-480-callisto-...ping-bags-pads
There is a method to my madness. I know that both of these bags are huge and bulky, but that doesn't matter too much for winter camping as I pull a pulk. I am planning to do some winter bikepacking as well, where bulk may play a bigger role. The main reason I am considering a modified Callisto bag is that my wife and I have a $100 REI gift card coming, which would pay for this bag. I know that I'll just want down eventually, so is this bag worth considering?

Other questions:

Is there a step-by-step guide for down UQ's?

If I choose to make my own quilt, where do I buy down (or primaloft) from?

Has anyone else been successful with a modified sleeping bag UQ in sub zero temps?

Thank you all in advance for any help. I've tried to search the forums, but there are some things that I just can't find answers to.