You can create a new view layer with material override.
The material will be with Camera Data node and mixing by the object index N.
Change the object indexes of all the particle objects to N, simply with Copy to Selected.
And now you can have better results than standard depth pass.
Check the file for more details.
The spider has a gray background that hides the image on the layer below. You need to render the spider whit a transparent background and the problem should be solved.
As the picture below.
If you don't know how to set a transparent background, go to properties editor/render/film/transparent
Do it in the 3d view?
import videos into emission shader planes and animate them.
I know it's stupid but I think it's the only way you can work graphically.
I wish that in 2.9 the Video editor thing will become Good!