Hi Xavier,
There is a good oppurtunity for you to view a completed boat in Maimi. Check out this Creekmore on Yachtworld:
http://www.yachtworld.com/core/listi...g_id=57289&url=
If the inner stay is high enough (Solent stay) then you don't need to back it up with a stay. If it's attached lower and the boat is meant to fly double headsails, cutter style then opposing support is likely needed. The key is don't do anything without more knowledge and we can't be much help without detailed info. But we do like to help. Get Brion's book, The Complete Rigger's Apprentice, read it cover to cover, do the math on your rig and it will save you many times the cost of the book each time you talk to a pro.
Garhauer equipment is well regarded on the C&C list. I have purchased deck organizers and they appear well made. Sometimes the finish detail is not really high end (but then neither is the price!).
Regards, Russ