Basically, the Solo boards are for those games that only require a single thread, regardless of number of players.
If your game benefits from more than one thread, you use the small games forum instead. For example, if you have a thread for the actual story, a thread for the OOC talk and a thread about the characters, you make three different threads, with more or less the following names:
[name of the game] In character
[name of the game] Out of character
[name of the game] Characters
People can see they belong together by the tag up front. If the total number of posts from all threads together reaches 1000+, you can contact a goddess to create a new board for you. Then you can contact a Satyr/Siren to move the threads for you.