Custom content won't be added to your game, Maxis defaults will be assigned instead. Not sure about lots and rooms, but this goes for the Sims in the gallery.
For example, you download an Elsa Sim with a CC dress, the CC will not be included. Unless you have the CC installed beforehand, the Elsa Sim will use Maxis defaults instead. AFAIK
There are some households in the gallery that I would like to add to my game but the contain custom content, if I add the household to my game will the custom content be added to my game or will it be like in sims 3 where it said missing content.