Barebones Assumptions for a Homebrew Universe

I'm starting a game with new players this Friday, and I'm looking to basically create a document that includes all of the setting "requirements" to give support to the mechanics, items, and rules of the game.

Things like that there has to be a galactic empire, there are only 6 alien races, jump ships exist, psychic powers exist, planets are mostly left to their own devices and have their own tech levels, and of course the golden rule that communication is only as fast as the ships. The basic, barebones skeleton of the universe.

There have been times in this and other games where I homebrew something and it ends up directly contradicting a basic aspect of the game. I'm hoping to run this campaign very long term, I really want to have a strong structure of what is required and what is not, in order to worldbuild so that anything from any book (Mongoose 2e) can be used and I won't have to readjust my entire canon to support it.

Any help would be appreciated!