"So I imagine you cut a slit down the middle of the pad holder on bottom of the bag and added the velcro?"
I'm not really familiar with the Big Agnes system, but if a mat is too big to fit in the sleeve, then yes, you would need to cut a slit to expand the sleeve and either add some extra fabric, or add velcro. We prefer the velcro because then we can mate our bags (WM PODs) to any size or shape mat, but either would work. The PODs had the same problem when they came out...they were designed to work with only one type of mat, and wasn't a a very flexible system due to this.
No, the RidgeRest would not be good down to 20F. That's when we use the DAMs, or you could just add another mat to your CCF to boost it's rating.