I have a school of 6 pajama cardinals. So far it's working out. Not all cardinals will school indefinitely, some will start picking off members of the group.
but don't cardinals just kinda hang in 1 spot? all the one's i have seen in the lfs just stay there, i thought they were supposed to be more active when the lights go out? maybe i miss-read
cardinals are nocturnal......Lotus' group hang out and are very cool......in a 150 like you have I would seriously consider a school or flasher wrasses or anthis......
i agree with lorna, try a school of wrasses or anthias, they have cool behaviors in groups, and for most wrasses and anthias the male of the school will take on some really cool coloring.