I do not have either the VoomPC case or the Carnetix PSU, so someone else may be able to better answer your questions, but I'll take a stab at it.
As far as compatibility, these two will work together. As far as fitting in the case, I don't see that as probable. The Carnetix p1900 is a board contained in a case that is the size of a normal AC PSU. It's in the case because it has a fan too cool the PSU. Granted, I don't know if the fan needs to be there or not, but I'm assuming it's there for a reason.
Theoretically, you could remove the board from the casing and try it out, but still, the dimensions of the board are much different from that of the M2-ATX. The M2 is long and skinny while the Carnetix is square-shaped. I know the VoomPC case has limited space available, so, even if you removed the board from the carnetix, you may still be in trouble when trying to mount it.
But, keep in mind, you don't necessarily have to keep everything in one small area. In my case, my Opus 250 didn't fit in my case or even where I am storing my PC, so I lengthened the wires and stored it in my center console with my PC under my passenger seat. So, don't be afraid to get creative.
Hope that helps or that someone else can better answer your questions.