In regard to light, your tank sounds just fine, either with the 175W or the 250W MHs.
Some other considerations:
Gigas clams come from deeper water than croceas or maximas, so do not require quite as much light. Under any wattage of MH in a standard-depth tank, they can be placed on the sand and receive enough light. They are found in the wild on sand, so placing them directly on the substrate is the most natural placement.
Because of the deeper water habitat, these clams do fine under the higher color temp bulbs (7100K, 10000K), whereas some will argue that the maximas and croceas need the 5500 or 6500K lights because they originate in shallower water, where they receive a fuller spectrum of daylight.
If you keep clams, especially one that grows as big as quickly as a gigas, you'll need to make sure your calcium is high and your alk is in the right range. Dosing heavily with kalk does the trick for me.
Fishes to avoid are any that will eat corals (even sometimes) such as angels and some wrasses (however, Coris gaimard wrasses are often kept in clam aquaculture tanks because they eat pyramid snails, which prey on clams). Snails that prey on other mollusks, such as tulips and murexes, are to be avoided.
Some recommend an occasional feeding with DTs or some other plankton substitute, as clams do filter-feed to some extent. Knop claims this is necessary, Tullock, Baensch, and Fenner do not agree.
Also, if possible, make sure you get an aquacultured gigas, as they are becoming threatened in the wild. They are harvested extensively for food in many eastern countries. If you go this route you will be unlikely to find one that big.
Much of the above is true for maximas, although they will need to be much closer to the light.
Keep in mind that a gigas that big might be a little too big for your tank. 13-15 inches is one monster clam (I have on about 9" long). A clam that big will suck all the nitrates out of your water (they absorb nitrates from the water column for food in addition to relying on their zooxanthallae and filter-feeding), and you might have to supplement the nitrates in some way. Also, it will be a BIG calcium draw, and you might not be able to replace enough evaporrated water to make up for the depleted calcium. A way to increase evap so that you can drip more kalk is to aim a fan at your sump. This will drop your temperature somewhat, so you'll have to turn up the heaters to compensate.
HTH
