I've found a way to set diplomacy.
There are some drawbacks odcourse, but if there is few civs it works better. I found out that if you want two civs to be hostile towards eachother from the start you should do two things.
One: Make thieir Governments very opposite to eachother. For example Communism and republic.
Two: Make the cultural groups very different. for example asian and american.


This works very well (at least it did for me :D :D )
As you see it works best in a mod\scenario with few civs. There is a limit for how many civs you can have if everyone should be enemys. You can aslo combine theese things to make civs halfway enemys. I hope it works as good for you as it did for me. Good Luck!!:goodjob: :goodjob: :goodjob:

Another way is to start your scenario as an MP game. Play all the sides and declare what wars you want. Then save it off. You can then play it as a single player game with the proper start conditions. (They may make peace in a turn or two).