"Usable once only" cards in combat

[GSGS]

Hello,
I would like to know if all "usable once only" cards are playable in combat? (when the card itself does not specify. )

We have run into a situation playing Munchkin Fu, where a player tried to play "Monkey Spirit Guide" in combat to take "Spirit mirror" from the discard pile. Since "Monkey Spirit Guide" does not say "can be used in combat" nor "cannot be used in combat", we would like to know ifr this move is allowed?

And since "spirit mirror" says "usable once only", is the player allowed to take it back from the discard pile with "monkey spirit guide"?

Thank you so much!