I think the lighting issue is debatable, I've kept clams in PC lighting from 64 to 120 and they've done fantastic, placement of clam to light (inches). Should it be done? not by someone who doesn't have experience with clams. Clams generally do better under MH lamps but you can be successful with PCs as well.
As far as keeping a clam in a 12...uh...no, to the best of my knowledge, every clam will outgrow a 12 gallon withing 2-3 tears not to mention your lighting issue. Clams require lighting and younger clams sometimes require feeding(phyto dosing etc etc.) which can alter your water params.
I hate to say this, but sometimes you just have to walk away from inverts that may not suit your set up.
Good Luck.