To keep you updated, I manage to do what I wanted. And if someone ever needs to do it, I'm going to explain how I did it.
What I had was an array with 40 X entries and a number value in each of them. I wanted to create given object from a family based on this value. For this I created an family instance number variable and assigned a different value for each object in the family.
But as blackhornet told us that doesn't work, as we can't create a given instance from a family based on a variable. The instance created will always be a random object type from the family.
It was possible however using rexrainbow 's plugin Nickname. Instead of defining a number instance variable for each object type in the family, I added a family behavior Nickname, and entered a different string (A, B, C, D, ...) for each objet type in it.
I then stock these values in the same array as previously but as strings.
Then the below event just creates an instance from the family (placementTrumpet) which has the same Nickname than stocked into the array at curX.
Exactly what I wanted to achieve, thanks a lot rexrainbow for the awesome plugin!