If the enemy chooses a 5 might unit with sacrifice, can I use star cross on it first?

No. The 5-Might unit is killed as an additional cost while your opponent plays Sacrifice, before you get priority to respond. You cannot "Star-Cross it first" to save it.

Here's the sequence:

1. Your opponent plays Sacrifice — during Step 4 (paying costs), they kill their friendly Mighty unit. This happens before Sacrifice is finalized on the chain.
2. Sacrifice is finalized and added to the chain.
3. Only now do you get a window to play Star-Crossed as a Reaction.

At that point, the Mighty unit is already in the trash, not on the board. Star-Crossed targets units on the board (Rule 355.9.a.1), so you cannot target the sacrificed unit. You'd need a different enemy unit still on the board to even play Star-Crossed (it requires both a friendly and an enemy target — FAQ #10191).

Even if you could target it, the unit is already dead — Star-Crossed returning it to hand wouldn't undo Sacrifice's cost payment. The FAQ on Sacrifice + Star-Crossed (FAQ #10993, #10982) confirms: the cost is paid during finalization, so returning the unit later doesn't stop Sacrifice from resolving.