If not does that mean the Custom Content houses must be placed before starting the game?
Yes, they have to be placed before you start the game.
Umm don't you have to start a game/Family before you can place stuff in lots?
Yes you do have to start a new game before placing a home on the lot. Here is a breakdown of what needs to be done.
1. Start a new game. When you reach the screen where you have chosen your town to play in and can start making your Sim/Sims, you will go to step 2 instead of creating your Sim.
2. Go to the options menu ( ... ).
3. Go to Edit Town.
4. On this screen you will choose the Show Empty Houses tab.
5. Click your chosen home from the library and place the copy anywhere you are allowed to depending on lot size.
6. Now you can click the button on the top of the screen that says Return to game setup.
