I haven't seen a guide per se, but that would be useful information. I have seen this discussed here quite a bit. I just look at the free bonuses and starting techs for each race to get an idea. There's also the number of bonus points a race gets and the class of its homeworld. For example, in Dread Lords, I like the Yor for their miniaturization bonus and good starting techs. In Dark Avatar, I like the Thalans for their Class 15 homeworld and industrial super ability. You do take a hit playing custom race. They only get a class 10 homeworld, no free bonuses, and less starting techs. I don't think it's one of the best races to play, but if there's something you really want to start with you can't get it with a predefined race, it's a way to do it.

Terran:
Typical diplomats with decent trading abilities.
In the early game, focus on tech trading, using your superior diplomatic capacities to swindle the AI out of loads of money, or even better useful technology. In order not to give away too powerful techs, some typical options are
- minor techs (e.g. sensors, the very basic weapons, extreme world colonizations AFTER you've colonized those particular planets)
- helpful techs to allies (e.g. trade, alliances, some basic weapons; economic / influence / happiness techs if you see your allies are having trouble in one of these areas)
I suggest to try getting technology in return since you should be able to have a good income from trading (Trade +25 and an additional trade route should help)
Research +10 may not seem that much at first sight, but it still puts you ahead of most of the field, so be sure to put this to good use (advancing technologically as well as generating techs to exchange with the AI)
If you want to go for a diplomatic victory, this race seems one of the obvious choices.
Remember that you don't have any military advantages from the start, so either research enough in that area or rely on diplomacy to keep safe.
I always consider them a good race to start out with, nothing too extreme that will push you too much into 1 direction. Solid grounds.

Korx:
Super Trader indeed, and their research & weapon advantages are nothing to sneeze at either... On top of that a big fat economical bonus (+45)
Get those trade ships out there as soon as possible to generate heaps of cash.
Since you start with Galactic Warfare, Ion Drive and a Weapons +20 bonus you may want to consider an early attack. With your trade income you could even buy/lease some military ships early on for even more of a headstart into the early war you're about to set off. Note that with your trade income & economy bonus you can impose lower taxes to keep your population happy; after all you don't have any Morale bonus at the start.
In addition, your Research +15 and Creativity +25 abilities should allow you to climb up the tech tree faster than almost any other race.
Another nice combination for a quick start: Stellar Cartography + Ion Drive for some fast colonization.
Me likes, especially since I'm much into economy + research strategies at the moment.

Drath:
The manipulative bit from the lot, with the best natural diplomatic abilities (+30)
All this talking is nicely complemented by a good morale boost (+20), Xeno Engineering and Xeno Industrial Theory which help you to set up production fast.
Since you also have Galactic Warfare, Ion Drive and Stellar Cartography, the early colonization -and to a lesser extent early attack- apply for the Drath as well. Keep in mind though that you don't possess any military advantages in the beginning so don't overdo the occasional early attack. After all your tongue should prove a far more dangerous and versatile weapon.
One of my less favourite races, but then again that might change with some good strategy tailored to the Draths.

Thalan:
Weird creatures with a fitting special ability: Super Hive for an increased range and faster building of factories. Fortunately enough it doesn't stop there.
A more than decent economy bonus (+25) plus Xeno Economics, boosted social and military production (+33 each) should support an early headstart.
Xeno Industrial Theory & Engineering will allow to turn these advantages into a military supremacy even if you don't have any weapon or soldier bonuses. A +10 miniaturization gives your ships a little extra zest.
Increased range combined with Stellar Cartography can help to colonize some distant planet(s) first.
All in all I'm slightly less impressed than one would imagine with these production bonuses and special ability. I guess I really need to give these guys a few tries.

More will follow when I happen to have some more time. It's all been said before but I hope this may form a more structured, summarized overview than 100 threads of dozens of posts each where you may go for more in-depth discussions.