Project Structure: adding Android, Spring, PHP and Static Web modules results in error message

Upon creating the project structure, I decided to begin with an "Empty Project" and added these modules:

• Spring
• PHP
• Twitter Bootstrap
• Google Web Starter Kit

After adding an Android module, IntelliJ complained about the project not being Gradle-conform. As a consequence, the Android module is not visible inside the project. Its files were added to the filesystem, though.

Afterwards, I tried to do it from scratch and the other way around beginning with the Android module. This way, it seems you can't add PHP or Static Web modules to the project.

Now I'm stuck, and since I don't know nothing about Gradle, there are a couple of questions:

1. Is there any way to add all these modules to a single project? How would I do that?
2. Is there an easy way to convert the initial Empty Project to Gradle?
3. Is there a better approach in general, should I avoid mixing Android and other modules in a single project in the first place?