Against the AI:
I like to play Proportions from a one planet, low tech start. I'll get as interesting a quadrant as I can, with NOT all warp points connected, empires allowed to start in the same system, NOT all player planets equal and warp points anywhere in the system.
I'll concentrate on a few key breathables (you sort of have to in proportions), and in my latest game I've taken the 80% planetary capacity trait which really adds challenge: It makes tiny non-breathables practically (but not quite) useless. My goal in this game is to build a ringworld.
I expand quietly, slowly and peacefully, concentrating on infrastructure in order to give the AIs plenty of time to develop the kind of economy/ weaponry that will provide me with a challenge later in the game.
If I find myself trapped in an isolated part of the galaxy with no AIs and no choice but to develop WP openers then so much the better:-) By the time I "break out" there will (hopefully) be a wonderfully advanced alien warmonger on the other side just waiting to give me a decent run for my money.
In these games I don't develop weapons tech at all until first contact or even first combat, and I try not to start wars (but usually finish them=-)