I want to create a tiny islands that are really close to each other. Some of them will be infested with Zerg buildings. The problem is that the creep will swim to the other island. I've tried using null terrain to control it but that still doesn't do anything.

1. How can u control creep using terrain?
2. Is it possible to have Zerg buildings WITHOUT creep (using mods?)

If you place a zerg building on something like a doodad or unwalkable terrain I believe that stops it from having any terrain at all, and stops creep colonies from producing any creep.

Depending on the tileset, you could use bits High-Dirt-to-Dirt cliffs mixed into the Dirt-to-Water cliffs. The height difference should block the creep IIRC.

I think placing them on water will remove the creep.

As far as I remember, there are awesome test maps in the DLDB, search a bit

Use preplaced Zerg Extractors at spots of creep you desire to remove, kill all Extractors at the spots you placed them at, and remove the geyser via triggers. This way, you can control the creep's shape even at terrain where buildings can be built upon.

With this method, you can remove all creep on walkable area. Or have the Extractor stacked on any Zerg structure, kill extractor and remove geyser to remove the creep underneath the Zerg structure.

Does this happen in game? I don't remember creep being able to go across unbuildable terrain without creep also being on the other side except in the editor or if creep has ever been there before.

Extractor removal is you best bet.

That is not possible. Even if SCMDraft says it will be on the other islands, in-game the creep is gone. The inaccurate creep display is a known bug of SCMDraft.