It is unlikely the pipe and the plate will have the same specification unless the pipe is fabricated from plate.
If qualifying the welding procedure specification, it is easier to select the materials from the same base metal groups. For example, SA53 or SA106 pipe and SA36 plate are all contained in the P1 group (ASME) or M1 for AWS B2.1.
If the materials are not the same thickness, the thickness range qualified would be based on the thinner material.
The joint used to qualify the WPS should be a butt joint rather than a corner joint. Based on the questions, you might want to consider if you have the back ground and experience to be developing WPSs without help from someone that is more familiar with the process of qualifying the WPS and the various welding standards that apply. No disrespect intended, just calling it as I see it. The welding codes assume the individual responsible for qualifying WPSs has the expertise (training and experience) to develop and qualify the WPS. The codes do not provide step by step instruction on how to qualify the procedure or the welder.
You might consider reading past issues of
Inspection Trends that include articles on the subject of qualifying WPSs. I believe there are several articles on the subject starting with the winter 2010 issue of
IT. The link to the first of several articles on the subject:
http://www.aws.org/itrends/2010/10/IT_October_2010/index.html#/0/ Subsequent issues of
IT discuss various aspects of developing and qualifying a WPS.
Al