Is there any way to use Manneken Pis with Rome (B)? Where is player with Manneken supposed to get Leaders' cards to play them?
Sure. There should be leaders left over so that they can draw from those left over as well. There are 36 leaders. Everyone starts with 4 so 28 are taken leaving 7 leftover. Rome gets 4 of the 7 so Manneken Pis would get the remaining 3. He'll get to place 2 of the 3 for a total of 5 but has less choice than the Roman player.
If you wanted him to have 4, then use the spare leader card or Stevie Wonder to give MP 4 cards to draw.