It's hard for me to imagine that you couldn't mount a Givi box to a Fehling rack. While I haven't installed a Givi brand box to a bike, I've installed top boxes on universal racks without any major hiccups. The Givi mounting plate would just have to bolt to a rack that is essentially a grid of tubes. If you didn't want to drill through the tubes, you could probably just bolt through some improvised straps under the Fehling rack. The box would obviously have to sit behind the upright part of the rack but I think that would work ok.
I gotta believe that the Givi mount is gonna sit pretty far back because of that " backrest " bar. Therefore I don't think I would mount a very big case due to the fact that it will be sitting so far rearward. Just my thoughts.
As long as you do not put too much weight in the box you should be fine. I use saddlebags for all my heavy gear and just put light weight stuff in the top bag. You may want to look into adding additional support for the Fehling rack. I have seen a couple of videos and articles where supports have been welded to the rack to strengthen it. The rack should hold about 4-8 kilos. Are you getting the side mount for saddlebags also?
The rack will bend if you put too much weight on it. I recommend using a drybag on it over a case. Less weight and more capacity. Just remember no tools or anything heavy.