Hi Valerio,
Here's what I know based on what I've actually done:
To use Assembler through an EJB interface, you don't need to install workflow. You just run the assembler installer and LCM and write the EJB client to your needs.
To use Assembler through a Watch Folder, you run the Workflow installer, then Assembler installer, then the Watch Folder installer, and then run LCM on all of them together. They all run in the same application server (JBoss in my case). Then you install workflow designer, deploy the QPacs, create a simple workflow, configure the watch folder to invoke the workflow, and start dropping files into the folder.
I have not run Assembler through a Web Service because I haven't needed to (yet). Assembler does not have a web service interface, but workflow does, so in this case it would look a lot like the watch folder setup except you could skip the watch folder installation and configuration. You'd still need to set up a simple workflow to invoke assembler, and then call the workflow through the web service.
I happen to have a full Workflow license, but I was told by a reliable Adobe source that purchasing assembler includes a "limited" workflow license so that you can use the watch folder and web service to invoke assembler. I don't know exactly what the limits are or how they are enforced. But the overview page seems to implicitly confirm this:
http://www.adobe.com/products/livecycle/assembler/overview.htmlI hope this helps.
Don