Making Certain Units and Improvements Available to only Certain Civilizations
Basically, modifications to RULES.TXT make this possible and it begins with the Technical Advances, and their various prerequisites, and the "technology tree" that results from them. It also involves the use of scenarios. Basically, you must decide the premise for your scenario, create it and the civilizations involved, and then give the civilzations involved certain basic technologies. Then in RULES.TXT, through use of the "no" option on technical advance prerequisites, you insure that the possible technical advances for each race are "blocked" off from each other. Once you have created these "individual" tech trees for your civilizations (and they can apply to single civilizations or several, depending on what advances are given in the scenario), then you move onto the City Improvements and Units areas of RULES.TXT.
In the City Improvements, Wonders and Units areas, just insure that the prerequisites for particular advances fall within the technology tree of the civilization you want to receive them. The same can be done with government types. You will of course have to deal with the units.gif, icons.gif, etc for new or modified units and improvements or wonders that you want to appear differently in the game.
Now, the only problem is civilizations being able to require technology outside of "their" tree through negotiations and conquering. With respect to negotiations and trading between civilizations. The EVENTS.TXT file available through the new Scenario CD gives you an event called NEGOTIATION where you can prevent one or all civilizatons from entering into negotiations. This can be used to insure that certain civilizations can not negotiate (and thus trade) with one another. Without the Scenario CD, you can keep yourself, the human player, from trading, but the computer players may still do so. To offset this, you can use the AI and CIVILIZED modifiers of the technical advance in RULES.TXT to make the advance less desirable to computers, but then it applies to all of them. This also is true for aquiring certain technologies via conquering. I have not figured out any way to avoid the potential resulting from conquering to date.
At any rate, this methodology works and can be used to accomplish the desired results. As with all modifications, there are peculurarities of the civ2.exe which will have to be dealt with as they arise and are discovered. Most of these are simply undocumented limitations or aspects of the game.
Hope this helps. Good Luck!