Hello, I can't remember what exactly I did, so can't offer much help - however you are completely right. I was able to use FMC addon in A300 without issues in P3D v3 and Windows 10.
The only issue you might run into is that font will be stretched and the CDU not responding to inputs, but that's easily fixable by installing windows 7 fonts. Google it and it will be explained somewhere.
As for the issue you are experiencing now - you are on good path. If I remember correctly you had to go to texture installer and choose which model to use for each livery - you will have to pick fmccargo or fmcpac for each.
However, if you have troubles with this, if I remember correctly you can edit aircraft.cfg file in the plane's folder and edit each livery section (or only the ones you want) manually. Edit those lines depending which model you want:
For cargo liveries:
For passenger liveries:
Hope that works, simcheck A300 is a total bae.