That may be easier to do in the iggy than in the patcher. Although maybe some other modes would have to be replaced for it to work, the game is not prepared for those modes.
Anyway, I'll tell you a secret to enable 2vs1, 3vs2, 3vs1, 1vs0, 2vs0, and 3vs0 (but not 1vs2, 2vs3, 1vs3)


Go to the internal folder of the installer and find the CharaSele.as file. Open with a text editor.
Search for the function "pushStart".

Remove this part of the function:

var _loc1_:int = m_callback.GetUserDataInt(ReceiveType_PlayerEnmNum);
if(_loc1_ > 0)
{
return;
}
Now, make an action that forces the installer to recompile the file. Reinstalling a character mod is fine.

After that, in game, for  2vs1:

- Choose 2vs2
- Choose the first two characters, and then the first one of the other side.
- Press the start button (xbox) or space (keyboard).

For 3vs1, 3vs2:

- Choose 3vs3
- Choose the first three chars, and then 1 or two from the other side.
- Press the start button (xbox) or space (keyboard).