You are free to mine Nether fortresses and abandoned mines in both Sanctuary and Frontier. Both of these are not limited in how many a world can generate so they are not exactly a finite resource. The only exception is the abandoned mines in the Sanctuary spawn village.
Strongholds and NPC villages are community resources, so we don't want to damage them without discussing any proposed changes. This is especially true for villages as their role in the game is still being developed. This is stipulated in the TOG Minecraft Server Code of Conduct
for both Frontier and Sanctuary.
While villages likewise can't be "claimed" by players, it is fine to build walls or fences around them (so the NPCs don't escape) and build additional structures next to them. Those structures will be regarded as your work, but the village will not. So if you want to expand the village, you can, but don't change the original village.