accelerated start scenario does what exactly?

You start off with extra cash and the first few techs of several branches already researched.

Agreed with Reply #1.

You have WARP engines researched, the first weapon techs up the Beam/Missle/Mass Driver branch as well as the first defenses up the Shield/Point Defense/Armor branch.

I think a few other techs are in there as well.

I tried a game playing this a few days ago and found that it:

1.) Would only allow me to play a game with all races (I could not decrease or increase the number of opponents)

2.) Nor could I go in and manually set AI intelligence.

This might be a bug. Accelerated Start is a good tool for learning gameplay mechanics but I found after about 50 turns I had this game won and so I deleted it.

The scenarios don't just modify standard games, they actually replace them. You don't get much for options with scenarios, sadly.