Each concurrently running instance of Poser needs its own license, AFAIK. Since the virtual machines (and the host) are connected over a virtual network, Poser will detect if another instance with the same serial is running.
Operating system: same story. Each virtual machine needs its own OS license.

Since VMware creates "virtual hardware", you have to install an operating system on each virtual machine. 
Or you can use one of the free preconfigured virtual machines that you can download from vmware.com. There are several Linux configurations available on their site, at least one including Linux, Apache, MySQL and PHP, fully configured.

As for Windows, I'd have to read the license agreements. MS also has a VMware-like tool (Virtual PC), and I know for sure they have changed the Windows licenses to incorporate situations where Virtual PC/VMware is used. Don't know by heart what the license conditions are, though.

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