A few questions.

[Kemystul]

1. Is there any way of modding a unit to require a specific social policy?

2. Is there any way of modding a unit to require more then one resource type (like iron & horses) or multiples of a single resource (like 2 iron per unit)?

3. Does anyone know if there can be more than 10 policy trees?

4. Lastly, has anyone figured out how to make a building provide a resource?

 

[Spatzimaus]

1> Not directly. You could handle it through Lua (having the Policy give a custom resource or custom technology and then having the unit require THAT), though.

2> Easily.
For multiple resources for a unit, you just need multiple row entries in the resource requirement table. While no vanilla units do this, I've done it quite a few times in my mod.
For multiple units of a resource, this already happens. Look at the Giant Death Robot (requires 2 uranium). The civilopedia entry won't say how many units are required, but the game manages it just fine.

3> Codewise yes, the XML can handle more branches. UI-wise no, not without some major modification, the Policy window is pretty much hard-coded to 10 trees. But there are a few mods out there that have looked into expanding this.

4> Go to the mod components forum and look up the thread entitled "Building Resources". We've been doing this for about four months now, although there was a brief period after the December patch when it didn't work. And it's not quite perfect, in that it only gives you the resource at the start of your following turn, but that's good enough for most purposes.

 

[Sneaks]

3. Absolutely. The trick is creating a separate context page for every policy after the 10th. In Procylon's Call to Power mod, he has 5 policy trees of 10 policies each (50 total).