I agree about the nipping. Sixlines have always been good for me but I have never had one with clams. These are supposed to be reef safe though.
As for the mandarin, I have read stories on RC about them fighting. They do share the same food source and would be in direct competition. A large system would be needed even if they were separated.
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,