The ambush racial ability says it allows Mentak to roll to destroy ships prior to space combat.

Question: Can the opposing player choose to eliminate fighters as casualties? It only reads as “ships” which seems kind of a duh! thing to say unless this is only to disallow card intervention. Any idea?

Yes, casualties from ambush can be taken on fighters. It's a good early game ability, but not useful when there are 10+ fighters heading at you.

Fighters can be used to take the hits for anything, with the exceptions of Graviton Laser System, the Mindnet Flagship, and Assault Cannon.