Yes, you can use any number of special cards as long as they are relevant. However, some cards have their own limitations on this (e.g. you can only use one Shinobi per base). Any effects those specials generate trigger immediately. However, any extra plays must be used immediately (you can't save them to see what other people do).

The 5-power Zombie Lord says, "You may play an extra minion of power 2 or less from your discard pile on each base where you have no minions." If you use Hidden Ninja to play the Zombie Lord, you may use its ability. However, this won't typically have a huge immediate impact on the scoring, as you have to play on bases where you don't have minions, and you already have the Zombie Lord on the scoring base.

This is covered in the most recent [rules for Pretty Pretty Smash Up][1], page 7:

>When a base has been chosen to score, **each player first gets to use any Special abilities that happen “before” the base scores**.

>If more than one player wants to use a Special ability, start with the current player and go around the table clockwise until all players pass in sequence (if you pass and another player uses a Special, you can then still choose to use a Special of your own). **If your Special allows you to play extra cards, you must play those immediately or not at all.**


  [1]: http://www.alderac.com/smashup/files/2014/11/SU7_Rulebook.pdf