Thanks to @ramones and @TELLES0808 for your replies.
In the way you suggested me it seems I have to set the value Health for each enemy everytime it's spawned, because it cannot be assigned a priori.
I would have liked that families' IV overrided the single object's IV so I could declare private IV this way:
enemy -> health = 1
bad_enemy -> health = 3
verybad_enemy -> health = 5
motherf####r_enemy -> health = 10
boss -> health = 50
and then apply a rule like
+Bullet: On collision with Enemies
-> Enemies: Subtract 1 from Health
+Enemies: Health = 0
-> Enemies: Destroy
But I understood it's not so...
Now, if I declare a family's IV Health = X, X will be the value for every health of every enemy. For sure indipendent for each enemy, but with the same start value and this is what I would like to avoid: the same start value for each enemy.
I suppose I can make a workaround assigning the Health value after spawning the enemy, but it's not an elegant solution: it's hardcoded and I prefer to modify a parameter of an object rather than a code line, if I want to make a change.
Or my workaround is the only way?
Thank you all in advance