OK - I read the FAQ 5.1 in regards to the core set #51 card "Stand and Fight" from the Spirit deck.

Does the "X" in the card cost area the variable that I have to pay to play a card from a fellow players discard pile? That is, if the Ally card requires 3 resource tokens, then "X" becomes 3?

Yes, that's correct.

what is the rule for solo play then? If i am playing alone, this card cannot be used?

"any player's discard pile" includes your own discard pile.