When casting light flies, my total leader (including my polyleader) is up to 2 times the length of my spey rod.
I don't think you have to get a custom line built. See if you can return the line you have and get a heavier one. If a scandi line is too light, I think you'll probably blow a lot of anchors.
I guess one way to add some weight to a line is to go with a long, 14 foot or so, polyleader, but normally on a 13 foot rod I'd go with a 10 foot polyleader.
Finally, I'd call the guys up at Rio
and ask them what weight line they would recommend.