You might want to check with support before getting xStreme. They have had problems with it and some of the packages it hooks up with. I've heard it can work with version X but not new version Y of a specific app, so you should definitely check on it before spending your dollars there. That said, I'm not an xStream user.

Of course, varying the type of trees helps, but there's also another way to vary trees. Change each one's leaf color. There's a bit in the manual about creating a custom textures based upon "Object Center"...see page 492 in the manual. A subtle color shift for your trees ought to create a less homogenous solution.

You can edit the leaf texture of an existing ecoSystem in the second tab of the layers panel.

Here's an example using the cherry tree plant which comes with Vue.
And here's the scene file if you want to see how the texture was created.