Manipulation of streams - help needed!

hello,
maybe someone has an idea for this question:

i try to feed one “custom preview” for baking multiple streams (5 streams, 5 materials).

i have 5 sources , where stream 1 is 1 surface,

and stream 2 has more elements.

how could i manipulate those streams to get an input in the “custom preview” like this:

stream 1 - 1 element / material 1
stream 2 - 1 element (but = 200 surfaces) / material 2

and so on ?
thanx in advance for any tip!

stan

image

stream 1:
image

stream 2:
image

I would go for something like this: