If an allied and enemy unit are selected with Star-Crossed and repulse is used in response to the selection of the enemy unit, does the allied unit still get sent back to hand?

No. Repulse counters the entire Star-Crossed, so neither unit returns to hand.

Repulse is legal here: Star-Crossed chooses your unit and no other friendly unit from your perspective (the second unit is enemy to you), so it meets Repulse's condition. A countered spell "does nothing and is cleared from the chain" (Rule 425.1.a) — this is different from the Retreat scenario, where the enemy target merely becomes unavailable and Star-Crossed partially resolves on the friendly unit (359.3.e.8). Countering negates the whole spell, not just the portion involving the Repulse target.