Here's some combos:
Baby's + Yoshi or Birdo: The Babys will get you back in the lead(5th or below) and carry extra items while Yoshi or Birdo will strike back with Egg power(4th and above) which makes them useful for the extra items that pop out.
Mario Bros + Yoshi and Birdo: M or L will strike ahead or behind with Fire while the Dinos can strike with also some items.
Wario Bros + Toad Team: Use The Toads for Speed and W and L for big damage to the opponent(s) ahead or behind you.
Bowser Team + Toad or Baby: Toad(look above) Babys(look above) while Bowser team can strike.
Remember, these are suggestions, not the best choices.