As far as I know, you cannot use custom graphics along with a custom mod all in the same scenario. Sounds dumb, and I think so too.
So, if you want custom rules and Sn00py's grahics, you will need to replace the Civ3 graphics wih Sn00py's versions (remember to backup the originals!). Then you'll be able to define any custom rules you want, like the AU mod.
I hope this addresses your difficulty.