Are we encouraged to di it?

I know Fallout 3: The Pit and GTA:Lost and the Damned are both in the database.

Are there any guidelines as to what should go in the database and what shouldn't?

I don't know of any guidelines, but I would suggest that DLC that is more like expansion packs (changes gameplay or adds a significant new mode) should go in, and DLC that is on the level of mission packs or new weapons should not.

Also, freely downloadable DLC (i.e. that could be called game patches) should be counted in with the original game even if it adds extra modes or gamplay.

These are my thoughts. Big DLC like the two listed in the OP are fine. Smaller DLC like the map pack in Killzone 2 are not.

However, you can just submit whatever you want, and let the game DB admins decide. They'll send you a note if you start submitting lots of things they don't like to deal with

Cheers both of you