It is a given that you fulfill as much of the special ability as possible. Therefore, if only one evil brigade is in play, Zeal can still be used. Similarly, Jepthah can discard one evil character in opponent's territory if there is only one.
However, in the case of Jepthah and other cards that have "cost" and "benefit" abilities, you must completely fulfill the "cost" to receive the "benefit." For example, you cannot use Jephthah's ability if you have 0 cards in your draw pile.
Kirk
