( imo ) no probs with t. derasa or t. squamosa with bulbs mounted parallel to each other ( four x 65watt = 260watts ) and with a mirrored reflector . if parallel they will receive light output from all tubes . lux/lumen output is constant along the length of a tube . four 65w. tubes mounted 2X2 would really only expose them to the lux/lumen output of little more than two tubes and i would be hesitant however it seems to be working for 2crazyreefers .
i'm guessing that mounting configuration is why pc's get a bad rap from people who generally use the watts per gallon method for determining suitability .
i've successfully kept t. derasa on a 4"d sand bed ( 18"d x 18"w aquarium ), under six parallel 32watt zoo med T8's for two years before replacing them with four 54watt ho T5's ( also have a t. sqamosa ) .
when i first considered a t. clam , A. Calfo told me my lighting would be o.k. for a t. derasa except the blue variety , of which i hanen't come across , other than a photo of a blue lined t. derasa in J. Sprung's INVERTEBRATES A QUICK REFERENCE GUIDE p. 124 .