maximas are more tolerant of water conditions than croceas, but in a small tank, any change (southward) of the water conditions is going to be hard on any clam.
IMHO, and IME, squamosa clams are very hardy and easy to care for. As project reef already mentioned, squamosas grow very rapidly, and get quite large. Derasa clams grow slightly slower, but can also attain massive sizes.
If you have your heart set on the bright colored clams--croceas and maximas, you have to buy the halide setup, IMO. I kept derasas and squamosas years ago under 4xvho's in a 40br, which is only 16" deep, and had great results.
Also, smaller clams need more nutritional supplementation---phytoplankton (IMHO) than larger clams. This may seem trivial, but I dose a lot of phyto, which might add excess nutrients to a small tank. Just something to think about.
If you don't already own a copy, the KNOP book on giant clams is excellent. I have an autographed copy (not bragging at all--I ordered it from a distributor, and it came signed---very cool.)