There are no rules about game names, nor should there be. This post is just my opinion, and shouldn't be taken as anything other than that.
Personally, I'd say it's okay to use a name which is also used by a ROM hack, or if it's also used by a game which is clearly dead (either a 6 month+ inactive thread or the creator has announced that it's over), or if it's also used by a game which is only in the initial "idea" stage and is clearly never going to go further. You don't need to have a disclaimer or anything. You don't need to change it later for any reason (unless you want to).
Of course, there's nothing to stop you from using any name you want, but don't be surprised if people denounce your game because you "happened" to call it the same thing as another popular game.
The most important part of choosing a name for your game is making sure it suits your game. All other concerns are secondary.