I don't know if you already have found the answer but here is what you need to know:
1. If you have purchased portal server from IBM and have ordered CDs, the packet will contain CDs for Rational Application Developer (RAD) as well. When you install RAD, you need to install portal toolkit (just check the checkbox in the installation wizard)
2. If you are downloading the portal server (as a customer or from the trial download page), the portal server download page has parts (download files) to download RAD. Alternatively, you could also go to RAD download page and download it from there
3. I have never used WSAD to do portlet development but I know that you can use it for that purpose. I believe there is one version of WSAD (5.1.2 or something) that comes with portlet development kit. You just need to install the kit at the time of WSAD installation. For older versions of WSAD, you need to install portlet development kit separately. But verify all this on developerworks. FYI, you can have RAD and WSAD running on the same machine. I have done this successfully before.
4. I don't know what you meant by "...need to install portal server separately and then plugin ... ???" If you are talking about the
test environment, let me know and I will give you pointers around that too.