If you want to spawn a rotated Zombie you can do so by:

/summon Zombie ~ ~1 ~

{Rotation:[90f],Motion:[0.04]}

If you do the same thing with a Slime:
/summon Slime ~ ~1 ~

{Rotation:[90f],Motion:[0.04],Size:1}

It does not work.

This issue persists even if you make a Slime ride something. If you summon a Slime riding a Zombie, the Slime will adhere to the Zombies rotation up to +-45° approximately. When the Zombie rotates more, Slime will no longer adhere to Zombie rotation.