We're talking a couple of different "skimmers" here. The skimmer attachment that some canister filters have (or you can get) will draw the surface water off slowly, and that does prevent the scum. I used to use these, but the tiny fish kept getting pulled in, and rather than making some sort of screen I just removed them.
The scum is natural, esp will no surface movement. It occurs in some tanks but not others. It is basically harmless, though unsightly. I get it in my Asian tanks but rarely in the Amazon tanks. In the former I too siphon it off during the water change.
I really doubt it has any connection to the type of food. Plants, organics, fish are more likely.