If I got it right, if I want to change a specific sprite, I should pick it singularly instead of the family, but where I need to address a number of sprites, I should use a family instead?
Not necessarily. Just because you have Sprite1 and Sprite2 in the Foo Family doesn't mean you have to pick Sprite1 by the family, nor does it mean you have to pick Sprite1 specifically in order to exclude Sprite2.
For example, Sprite1 and Sprite2 are both in the Foo Family. All Sprite1's have the value '1' in their 'myNum' variable, and all Sprite2's have '2' in their 'myNum' variable. You could pick all Sprite1's like so, if you wanted to:
+ Foo.Value('myNum') is equal to 1
-> Do stuff to Foo
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It all depends on what you need Construct to do.