The random teams feature currently only lets you set the size per team. That is, you can tell WarLight to make teams of 5, but it doesn't let you tell it to randomly divide into two even teams.
One thing you can do is to not join the game right after making it. Instead, wait until you have 10 people joined and join yourself to start it. That way you can replace players who decline.
I agree it would be a nice feature to allow specifying the number of teams to randomly divide into, which would make it give you a 4v4. You can vote on this here: https://warlight.uservoice.com/forums/77051-warlight-features/suggestions/1090501-random-teams-with-number-of-teams-param
I'm also considering adding a "don't start with uneven teams" flag that game creators can specify.