Maarten wasn't allowed to use his KO 6x6 at the V-cubes comp this year xD
This was in a thread a bit ago. Ton said there was nothing wrong with using it. Its essentially the same as using a mefferts/es 4x4.
http://www.speedsolving.com/forum/showthread.php?t=16391&page=2
Yes, although Verdes disapproves, naturally, there is nothing to say in WCA regulations that he cannot use this cube.
Feliks: DS cubes are pretty good for siamese, that's what I used, although you can buy 10 for like $12 free shipping somewhere on Ebay. I'll try and find the link for you!