In principle, you would have to capture the index of an instance so that you can compare this value with a value of your choice after instantiation.
Unfortunately, all the attributes that you would capture in the instances before instantiation are no longer available after instantiation.
Therefore, you would have to go the other way around, and capture the index in the points on which you instantiate your objects.
You then use this index in the node
Instance on Points as Instance Index.
The great thing about this is: This value is available to you in this form even after instantiation, so you can easily create a selection afterwards and apply a random scaling to instances with a certain index: