As far as I recall (it's been a year or so since I had to do that sort of configuration) you can do that. I assume you're asking because the only options you see are some other WAS versions? If I recall correctly you have to run something externally to install WAS 6 and make it known to your RAD. That would either be one of the install CDs or one of the downloads from the IBM website.
Yes. When you install WebSphere you can choose which test environments you want. You'd choose the WebSphere 6 one. You'll also need to make sure your project is using Java 1.4 and Java EE 1.4 since that's what WebSphere 6 supports. (6.1 supports something later.)