I just happened to check in...
First, Mbunna is one of the most knowledgeable sources of information here. Period. You should thank him for spending his time dishing out post after post of factual info (for free), rather than argue or accuse of following any crowd.
Second, yes you can keep tridacnids under fluorescents (VHO, PC, or T5) - IF you place them close enough to the bulbs. However, croceas and maximas in particular look entirely different when placed up high close to the lights, instead of down low on the bottom - unless you have a really, really plain clam. Thus, placing them high in a tank really defeats the whole point of buying such nice clams in the first place. Take a look at most of them sitting up high from the top of a tank and then you'll wish it was on the bottom...
Third, every clam is different, and there are instances where one clam may thrive under fluorescents at a given depth while dozens of others (of the same species) may slowly starve under the exact same lights and depth. Thus, there will be people from time to time that really do succeed at keeping high-light clams (like croceas) under fluorescents at a significant depth. But, when you buy a clam there's no way to know if it's a "tough one" or not. So, you need to provide an approproate amount of light for any clam of that species, rather than take a chance on perhaps getting an individual that can survive on less than it's cousins.
Fourth, it is true that clams can take many months, rather than days or weeks, to starve to death when kept under insufficient lighting. So, anyone that hasn't had one for about a year or more cannot claim success, unless there is obvious shell growth.
When it comes down to it, you can't go wrong with MH unless you've got a really, really deep tank. Using anything else can be taking a chance when it comes to most clams, croceas in particular. Still, I hope to soon see some real data from controlled experiments comparing T5 to MH, which may be enlightening (har, har), but until I see real data with comparable numbers, this reefer won't be taking any more chances than absolutely required.
James Fatherree
http://www.fatherree.com/james/bibliography.htm