when I choose a Scenario (e.g. Battle of the Gods, Accelerated Start), there are always 8 other races in the "Choose Opponents" screen, and I cannot remove any.
Hmm. I don't remember not being able to choose my opponents. But then it has been some time since I have played DA, and even longer since I played any of the scenarios, so I'm not really the one to comment on this.
I can, however, comment on the scenarios in TA. Since there is no multiplayer in GalCiv, not even head to head, there has never been any way to judge the relative strengths of players. The new scenarios are an attempt to rectify this situation.
Everyone starts out on an even footing. We are given a custom map, our opponents are set, our own race is set; and from there we are free to play the game as we see fit. This is by no means a perfect solution; the scoring is still heavily skewed towards a military victory, so those who routinely play this style have a distinct advantage, but it is the best we have at present.
Frogboy will, AFAIK, be the one writing these new scenarios, so perhaps he might be persuaded to mix up the victory conditions a bit, in order to make things a bit more interesting for those players who don't always play the game of annihilation. There will be a new victory condition upcoming in TA, and I think that one would be an excellent start, since no player will have any experience along those lines, and all players would truly be starting from scratch.