ORIGINAL: Gelatinous Cube
Well, it depends on the victory conditions. I've tested several games with different conditions, and it seems to definitely affect the way the other empires behave. For example, if you set the only way to win as 100% Territory, the Kiadians will certainly declare war on you if you are at a disadvantage. However, if you make the race victory conditions the most important aspect, they will strive to meet those goals. Kiadians will try and ally with you, and will defend that alliance to the death. Boskarans, on the other hand, will attack the crap out of you in order to reach those goals.
I usually play with "no victory condition" as i love to roleplay my empire and try to behave like a real space civilization. If you are true, the fact that victory condition impact AI behaviour may be disastrous for my type of play. An AI with no goal ?
It is quite disturbing.
Well, if you want a Sandbox you'll get a Sandbox, I guess. If you really just want to play a game that never ends, and yet the enemy is still competitive, you can fine-tune that in the victory conditions. It's not worded very well, but the victory conditions can be almost anything you can dream up. Setting Economy/Territory/Pop to very low values (but probably still something) and then checking "Race Specific Victories." Make the Victory Threshold 100%. It is very hard, if not impossible, to achieve 100% victory solely on Race conditions. Some of those conditions are brutal. But it will give the AI very roleplayable and worthy objectives.
This is all, of course, my speculation. I am hoping for an official in-depth explanation of this.