Hey guys I just played my first expansion game and i have this question:

when you play a leadership card that gives you a hero, and you don't have the unit required for the hero, can you keep the card until you produce that unit? also can you choose not to play your leadership card at a certain point and save it for later in the same stage?

The hero unit itself comes directly from your unbuilt units, even if you do not yet have the ability to build one of those types of units. Page 10 of the Brood War Expansion rulebook states,

"When a player gains a hero, he takes one of the figures of
the appropriate type from his unbuilt figures, places it in
any friendly area, and places a hero token beneath it...

...If a player does not have enough
plastic figures of the required type, he is allowed to destroy
a friendly unit of the appropriate type so he may gain the
hero."