Can this trick be adapted to just opacity?

[Joolsey1973]

I have been experimenting with this great expressions trick, of controlling the size of vectors with a gradation. 

https://www.youtube.com/watch?v=1UaZNcVobhg

I have a multible layers but want to bring them in one at a time without having to key every object, and I

was wondering if I could do it in a simular way to this guys tutorial? Or is there simular trick? 

Thank you. 

Julian