The choice for this is ultimately up to you, so no, it's not against the rules. For giveaways with multiple games or DLC the giveaway creator does have the option to request a reroll to a new winner if the winner owns the most of the bundle. Often times you'll find people will ask in the comments if it's okay to enter for just the DLC etc. etc. so mentioning that it's okay or not okay to enter for just DLC in the description will often help let the SG members know.

I saw your question above about if there's a way to check for DLC ownership. SG can't detect DLC due to the Steam API so anyone who owns the base game will be able to enter even if they own all the DLC. Some DLC can be seen in a Steam library without needing to add someone to their friends list, but there are occasions where you'll need to be friends with someone to see if a DLC is owned or not. For this particular game as I mentioned I own all but the Season Pass. Since we're not friends you could look at my Steam account and see if you see those DLC that I have, if you can't then you obviously will need to add the winner to your friends list to see it and check to make sure they don't own all of the DLC.