Is a 6" carpet seam okay?
As long as the add on is more than 1.5&ndash;2 inches wide, it will make a fine extension.  Much narrower than that and the seaming iron and seaming glue will make a bit of a mess.  Someone very experienced can probably manage it well enough though to make it turn out all right.  The six inch piece will be invisible if professionally done.

If you have any concerns, have a reputable installer or have a written agreement that they will do whatever it takes to get the job done to your satisfaction.