Are questions about mods to game on topic?
For example "How can I play as infernal in Fall From Heaven II" for Civ IV.
I don't see why not? A Mod like "Fall From Heaven" is essentially another game that happens to be built on an existing one. And we don't exclude games based on platform or genre in general, I see nothing about this specific case that would be problematic.
As for other mods, we've had plenty of questions like this one about mods to improve the baseline experience of a game - and I certainly don't see why they wouldn't be germane to our mission.
Questions about mods should definitely be on-topic, whether they are prominent or not.
Questions for all mods are on topic. However, if the mod or the game is too obscure, we may not able to answer you. Give a download link if possible to help us help you.
Questions about mods are off topic. However, the following should be not posting:
Requesting for a mod to use.
If it's Minecraft mods, don't ask question about tech support for modded Minecraft. only ask question for vanilla.
Mod making. Please post that at gamedev stack exchange.
Other rules the post violates.
So far, yes questions about mods can be asked.