The obvious answer would probably be "to create a good story", since multi-race starts are often designed for RP potential and are frequently encountered in the AAR forum.
In reality, story or background is the less painful and over the years when I wanted something quick and dirty I simply used either the real story (you can pick it up at any time and note the social/political landscape of that time) or simply the classic "what if", which would may require a bit more of your brain, but not that much. The final option would be going full brain mode (and patience) ON and get it from scratch.
However, I'm a bit curious if there are other goals that are put into this, when you start designing such a start with multiple races controlled by one human. Is it an inevitable slugfest towards mutual annihilation, do you take precautions to avoid this,
I don't find the goals to be any different than when you start exploring and find NPR to deal with to be honest. I am writing later some suggestions to make your game enough variegated or to keep it interesting.
or does the game inevitably end before any of this because of game slowdowns? Something else entirely?
I think if you never started anything like this, probably at the beginning you have many "fears", some of them may be justified (slowdowns), some others may come from random bugs people encounter, however version 2.3 just out is pretty much breaking bug safe and really code optimized for best performances. While many would disagree, for longevity I always use the same precautions: no civvies, I will control any new NPR. Spoilers are enough CPU varied opponents especially if you start as TN or Low Tech with Limited admins. When I play with Civvies and NPR controlled by the CPU I never Multi race start, I rather go solo. Also, I usually do that to try new things against unknown opponents.
A reason behind the question is that I have always been fascinated with the idea of multi-race starts, but have so far been a bit intimated by the set-up process and the prospect of game progression relative to the time I have to play. However, recently I have gravitated more and more towards trying it out, especially since the new templates decreases the setup when ship designs can more easily be replicated between races.
The fastest way to start is to design all components, ground forces, and ships without spending too much time on the names and such. You can edit this once you have completed your setup. In the past I would try to get these right from the start and was a massive waste of time. A lot of the things will unfold as you play, since the direction each race will take on tech tree will also determine a lot of the decision you'll have to also take as Game Master. For this reason, for Newbies in the Multi Race space, I strongly recommend to start early TN. The best for you would be:
Biology/Genetics - No Advancement (SM remove all researched techs)
Construction/Production - All Techs =<3,000RP
Defensive Systems =<1,000RP
Energy Weapons =<2,000RP
Ground Combat is controversial. Personally, I think you should know all the elements from the start, and leave only advancement and perks to the research tree and I set it that way. I leave the Xeno for once I discover ruins.
Logistics =<4,000
Missile/Kinetic =<2,000
Propulsion <=2,000 no Consumption advancement
Sensor <=2,000 no Gravitational advancement
Set your Research Points and Construction points to 0 as well. Let the needs drive your story. You will still setup an initial Commercial and Combat Navy, but I would keep it to a standard for all factions and let the randomness of the Shipyards dictate your designs. You should allow yourself one design per Shipyard and assign it from the beginning. The amount of slipways could determine the amount of the ships. I use the 10:1 Aurora rule. 1 Commercial slipway counts for 10 ships, while 1 Naval slipway will deliver only 1 Ship. But hey, you can make your own rules. For extra Randomness I design 1 dedicated Race per Race, without using the Human recommended. So I start with a Standard Human SM Race, then create a new race on Earth and call it Human Chinese, Human European, and so on. This will give me random traits and random bonuses/malus to the racial traits, meaning a race with 2.0 research, may need less labs than a 1.0 one or could simply be the opposite and just pump out research labs, up to you. This will be also potentially driving your goals due to the difference in traits of each race.
I will likely have several questions, which might be better off in the other questions thread, but for an initial discussion there is one particular thing on my mind. An obvious start would be the classical 3 faction Sol start, but part of the appeal for me is to experience warfare between multiple human controlled races. If the main base of all three factions are on the same home planet, it seems that any warfare would inevitably lead to mutual annihilation, unless heavy handed RP ensures that attacks on outer bases/colonies does not immediately translate to all out ground war on Earth. This is not a terribly unrealistic prospect, but not necessarily an interesting one, hence my question (to those of you who have experience with this) if this is just the nature of the game for such a setup, or if you are consciously steering the story in a way that avoids this? Or does the game generally slow down and stop before you ever get to this point, so that the enjoyable parts are rather the buildup instead of the actual warfare? Also feel free to suggest alternate interesting setups. I had considered doing a Mars-Earth two factions start and while it will solve the issue of immediate ground war, it would like still result in a complete stalemate once hostilities break out.
SM the diplomatic relationship and set it as fixed 10,000 so that you can RP the relationships and the treaties regardless. I know it could be interesting to have random races going to war because they do not like each other, but I still like to be in control and perhaps allow myself of a cold war sometimes.
Personally, by following where Aurora takes me, I hardly find the need to go war early on as the question is: why would this race do that? Yes, there is the dominance, needs of minerals, ideology, and so on. In reality, you will find out soon enough that your biggest enemy is the branch splitting where each faction goes its own way, as space is so big after all. So if you not careful your game will be a sort of this:
Race build up
Race discovers a jump point
Race travels and finds a suitable world
Race starts moving its people and infrastructure to the new world
Race goes its way on its own branch
Player have 4 solo games against CPU at the same time
I leave to your creativity how to avoid the above cirle. I have a couple of things that I use to keep races in check, but again, it's your game, see where it leads you or where you lead it!