i had my grey bamboo with two cali rays, but i later seperated them because the cali rays should really have a cooler water system to thrive, id id keep them for quit awhile though.. most rays need large systems as well,, the biggest problem with the shark ray combo is that you need a large tank because the rays need alot of open swimming space with little to no rock, but all of the bottom dwelling sharks need at least two caves IMO to hide in and the caves have to be rather large to cover their whole bodies...thus making you have a very large tank to suit both of their needs..if you search my name you can see how i had my 300 agressive setup..
you shoudl also tryt o get an 8 foot tank so they have the open room to swim,, mine swam alot and if elt sometimes that evan the 8 foot wasnt long enough for him to really go fast sometimes, so i think 6 would be way to tight but once again IMO