[Suicide Junkie]

1) If a tech is disabled, the AIs will all just skip over it as if they had never researched it and pretend they never wanted to.
Be aware that disabling a tech may mean an easier time against AIs, rather than harder, since they would now have fewer options for defense.

3) Since races are chosen randomly at startup, there is a chance that you will get all original races. You can temporarily move the races you don't want to play with, or you can generate multiple games until you get enough new races.

4) There are plenty of modders web pages. Could possibly be mine. IMO, P&N is great, since that's the way it was designed.
If you like the idea of newtonian mechanics, and disadvantages for every advantage, and lots of defensive options, then you should like P&N.
If you are looking for a really challenging game, playing as a pirate (or a nomad) against AIs can really be tough.
I also host the Image modpacks, which have about a 100 megs (unzipped) of new images of various categories.

5) Difficulty = high means that all the ministers are used.
Lower difficulties mean the AI gets crippled.
The bonus settings multiply the AI's production of resources and research/intel. Setting that to low (or medium) is common, since it allows the AI to catch up with human micromanagement.