I use OPC and know that it works with 1.5.6a.

I have not used FEC since it's plugin page says it supports through 1.5.5c. That does NOT mean it might not work with 1.5.6a, but there are so many changes in the new ZC version that I would certainly use a test site before loading it live.

Also, I can not find a github repository for FEC. Of course, this doesn't mean there's not something in the works. However, there are unanswered questions in the support forum since May.