how to specify line thickness of grouped object

Because I do not like a function of the arrow mark addition in the Inkscape, I perfer to using combination of triangle with a line. And , after that, I make these "grouped". Once the objects have been grouped, I think that it is impossible to change the thickness of the line by using control+Shift +M command. When I try to change the size of the grouped object, the line thickness is associatedly changed, which is not convenient for me. Do you know how to change the thickness of the line by using "deformation" (cont+shift+M) commnad? Thanks for your efforts to create this wonderful software.