@bemandrill About the extension, I recommend not keeping just one, put all possible ones, including upper and lower case, such as:
<extension> .md .MD .bin .BIN .zip .ZIP .gen .GEN .smd .SMD </extension>
The problem is defining the cores and priorities. It works better, when I include all cores from all systems with the same priority 1
I didn't even know it was possible to set core priority. Probably your priority problem is related to the fact that the original configuration theoretically has priority, so it makes sense to have to define that you want to choose the priority on your own.
In platform I have snes, but I dont know waht I can write.
Try "sf"as did you put in <name>, or try the names of the platforms you are copying from, separated by commas and spaces <platform>snes, megadrive, gba</platform>".
https://emulationstation.org/gettingstarted.html#config
