It might be possible to make the game winnable in this style, by modifying the AI diplomacy files. As things are currently, any AI will get angry at you just for being larger. Now for a certain type of psychology that makes sense. The militarist always regards anyone stronger as a threat. But for other types this is not so. 'Friendly' races ought to be interested in getting into treaty relations with larger and more powerful empires as long as those larger empires are not aggressive. If the AI would react properly to requests for help you could even get them to fight the aggressive ones for you.
As the AI gets more sophisticated maybe it will be possible to win a "cultural" campaign instead of a military one.