I assume they don't need swimming room so really if it can stay big enough to hold them. Lighting is one thing feeding is another. Arn't clams filter feeders? I'm not sure what they eat or how but you'd have to look into that and supplement it as needed.
In the 12dx yes, but not in the 24 without changong out the light to MH. The 12dx has 48w over about 8g of water witch = 6w of light. the 24 has 72w of light over 20g of water plus more hieght = 3.6w of light and the added hight. My squamosa did OK in my 12dx, but it is doing way better in my 50g under a 400w 14k MH. If I where you I wouldn't put any clam in a 24 NC with out upgrading the light.
Don't you have to feed clams stuff like DT's? I hear from the coralsdirect.com forums that cyclopeeze is too large for clams to consume. If you really want the clams to do good, bowl feed them when they are small and then give them MH when they get 3"+.
Same goes for mussels.... what the heck do they eat? I have a mussel that has about 50 zoas on it. He tumbled around for about a week after I put him in the tank, then I put a small powerhead pointed towards the corner and I sat him in the corner. He seems fine now. He's only in a 10 gal tank with a 96 w. p.c. I feed cyclop eez daily with an assortment of other stuff in it, like phyto plankton, mysis, formula one, and chunked up pieces of those white square chalk looking blocks they sell at the counters of most LFS's. I think the chalk looking blocks are calcium, strontium, etc, trace element blocks. Anyway..... I think my mussel is doing fine. It opens up when the lights come on in the morning and anchors his foot down and then late in the evening, he lifts his foot and closes up. I've had him almost a month, but I can't really seem to get a definitive answer on what they normally eat. He hasn't died yet, but if he's missing something in his diet, I'd love to fix it. Most LFS people I ask have no clue....