What are expansion joint requirements for magnesium oxide wallboards?
The best place to go for this sort of question is the manufacturer.  I deal with National Gypsum, USG, and others ALL THE TIME for questions like this.  Typically, the manufacturers web site will have all the design criteria you could ever want - if they don't, I would look for another manufacturer who took the time to make that information available.  These guys are generally very willing to help as a seccusful installation always looks good for them.  Give them a call.