The Windcutter 11/12/13' s are available as an uncut line without tips. If you want a line that will make the rod a tad slower than it is with the 10/11/12 Windcutter you picked up, buy a 11/12/13 Windcutter by itself without the interchangeable tips and cut it 15 1/2 to 16 ft. from the tip and install a loop. Then use the tips that came with the 10/11/12 Windcutter. It will work just fine.
The rod will cast the 10/11/12 Windcutter fine, I wouldn't buy another line for it for a couple years. Then I would buy the 10/11 MidSpey with tips. If you want a tad slower feel to the rod, pick up a 10/11 MidSpey without tips, cut it 15 1/2 to 16 ft. from the tip and use the tips that came with the 10/11/12 Windcutter.
I love the GrandSpey line but I don't recommend it for beginning spey casters. It is a lot of belly to keep aerialized, and it takes time to learn good spey casting technique to keep in moving and forming good "D" loops.
You are going to have a very powerful rod when you finish building it. It will be a winter stick that can throw heavy tips with ease.