I know that FIFA games use memory caching for some resources (models and maybe textures).
It means that the memory, allocated for one resource, can be later re-used for some another resource of the same type.
If it happens that the memory size required for that another resource is larger than allocated memory from previous resource - the game normally will crash.
That's how it works in FIFA Manager, and I believe it's same in FIFA07-FIFA10.
It will be a reasonable to ask how then original head models work, because they have different sizes.
All original head models are stored inside zdataX.big archives, and they are also compressed.
The size for memory, required for uncompressed model, is stored in fifa.fat.
And if you check fifa.fat, you can find that "uncompressed size" for all head models is same - even when originally models are different in size.
To solve this problem in FIFA Manager, I align all model files to the same size. I also replaced all original models and aligned them too.
And what could be such big files?
I think both models and textures.
FIFA 07 faces must be much simplier than the head-models you used for FM
Yes. FM uses FIFA09 models.
FIFA09 uses 4 different shaders (materials) for head model (head, eyes, haircap and hair) - FIFA07 uses only 2 (head and hair).
FIFA09 uses 2 textures for hair (color and a special texture with transparency masks), FIFA07 uses only 1.
How did you manage the viking head?
I made an experimental asset loader for FIFA07.
The model works in the match (at least at match beginning), but it crashes in Player edit menu.