A few KOF versions you mentioned I tried and could not get it working. I also tried to get Metal Slug 3 working (by combining some romsets), but it ended up in a segfault

I tried some of those games in the FBA emulator (which supports NeoGeo), but they crashed on me also.

How did you come up with a 40MB limit? I read the largest Neo Geo cartridge was King of Fighters 2003, at 716 megs (89.5 MB). Afaik gngeo is also not limited to 40MB, but I am not sure.

BTW: I now have my own gngeo version compiled, and I already added some extra key configurations, including savestate and loadstate. But I still have not fixed the crash issue in fullscreen after a few evenings trying. I hope to look at that further when spare time allows me that.