the only prob i could think of with being in the baby food jar is that it might restrict his feeding while in the tank, though it sounds like the best method for direct feeding since it will probably disturb the clam the least
as far as telling which clams are which, compare the byssal openings. maximas have openings that are way smaller than croceas, though im not sure how apparent the size diff will be while the clams are this small. compare the openings, if two of them are much larger than the other two, then youve got 2 croceas and 2 maximas, if theyre the same size, then 4 maximas