I applied for NEXUS after getting GE. I was approved in 2 days.
I did have to interview and get fingerprinted by the US person, even though they have that on file from GE. The Canadian and I just talked hockey during the interview. All and all, really informal doing NEXUS after GE.
FWIW, YUL didn't have GE kiosks in the fall. Not sure if that's changed or not.