As a new player who has just started his third game, I'd suggest Easy and Small settings, keeping the galaxy settings at default.  I'm trying a Medium sized galaxy with habitable planets Abundant, and I'm not sure I'm familiar enough with the game mechanics to deal with the micromanagement of that many worlds effectively. (Though I'm going to stick it out because it's an interesting galaxy.) I did leave the number of opponents at five, which was the setting for Small. 

A question for vets:  Are there default settings for number of opponents for each map size?  Or do you just set it to what you please?  I found Small crowded with 5 opponents, and now playing at Medium with the same number of opponents is more to my taste as there is a longer colonization phase. 

In any of these sorts of games, I like some breathing room for better development of my civilization and the AIs.  "Axe-rushing" in Civ IV is effective, but dull since the AI can't really cope, and so you take out 1 or 2 AIs early and get the advantage of the resource-rich starting spot that was their capital city.  I can see that there's no equivalent in Gal Civ II as Planetary Invasion is not a starting technology.

Ah, yes.  The "Rush" strategies.  The first one I recall was the  Russian "Tank Rush" way back in "Red Alert".

In MOO2, the quickest victories were in the smallest galaxy, taking Telepathic, rush building/buying a cruiser (just big enough for telepathic takeover), and getting to the home planet of a production-oriented race before that race could defend itself.

In DA, the addition of the Extreme planets made a great difference, IMHO.  As long as there are races in the game that begin with those special colonizing techs, the player cannot easily out-expand those races.  If the player takes one of those races and the other is not in the game, the player seems to have it easier in the early colony rush.  SEIV consistently achieves the effect by making each race choose a starting atmospheric environment.

I'm playing the largest galaxy, all abundant (and scattered), the max AIs, and the max minors, SLOW tech.  I've worked up to the level just above "Tough" and it is beginning to seem repetitive.  Initial expansion, grab what mining bases I can, run down the research tracks to stock exchange and manufacturing centers, picking up  a few other improvements along the way, throw out an extra few survey ships, change gov't once or twice, then dive into Extreme Colonization hard.  My intent is to get enough economic base to fund the next colonization wave, starting with the types no other race seems to be colonizing.  In some games, that's all 5.  So, as the Banks or Stock exchanges get finished, the colony ships start building.

Sometimes I have to bribe a race into a war or two to buy time but, once that second colonizer wave is launched, the game is essentially over due, in part, to planet flipping. The thing then is just to choose which victory I want for that game.

I'm guessing that the higher difficulty levels will give the AIs enough bonuses that they are able to better contest in the expansion phases and/or turn on me before I get that second wave of planets built up.  The AIs already are getting Invasion before I do, but seem still unable to mount an escorted invasion soon enough.  I suspect that will change.

I like Gigantic and Immense maps with 9 majors, slightly above-normal habitable worlds, and tight clusters. But I'm a 'turtler' at heart, so if you like an aggressive game, you might prefer more worlds, loose clusters, and/or just a Large map. Again, you're really the only person who can give you an answer to your question unless you are interested in top-ranking Metaverse scores. If the latter turns out to be true, Motti has plenty of peers who can help you on the way to scoremonster status.

GW Swicord:  Thank you.  Actually, I'm a turtler myself so similar settings should work for me as well.  Might set Habitable planets to common or occassional, though.  Not worried about score: I think a game that simulated the GC II backstory would be fun to play which is why I wanted all of them in play at the same time.

MottiKhan:  Thank you.  If I can't have all of them, I guess I can randomize who is in the game when I give this a try.

Edit: Sorry about the text color, but for some reason it will only let me change the last sentence to black.