I apologize if I come off as rude or mean-spritited, but I don't really see the benefit of forming an All RDM LS. I could understand the practicallity of making a BST LS seeing that BST could use that as a median to find advice, duo partners, or even others to camp NM's and HNM's. But RDM's are very different from BST. It is true that they are able to solo very difficult monsters, however, the fights take forever and a RDM is unable to tank the harder monsters like HNM's. And throwing some 18 RDM's at a mob wouldn't exactly improve the odds of winning that fight like throwing a well-rounded LS at it instead.
Using it as a mentoring LS wouldn't be that bad, however, I doubt anyone would wear it enough to actually find help or help someone.
With that aside, I beleive LS's are best made not by the goals and desires of its members, but rather the friends and companions themselves. Making an LS to perform a certain task is pointless if you don't enjoy the company of your fellow members.
With that said, if you could find a great group of RDM's whom you love to hang out with, go ahead and make an LS with them.