Salty,
On about half of my 30 EarthTainers, I have left the moisture barrier off, and instead just applied a half-inch layer of bark fines:
We are in a dry climate here, and if I do not use a layer of bark fines, the top of the potting mix dries out too much. The "blanket" of Fines tends to prevent this.
So yes, you can certainly mix in additional plants along side the tomatoes.
Raybo