In reply to:

Moose, the beast, is both plural and singular. Mousse, the foamy stuff you put in your hair, is also plural and singular. Perhaps it is just a matter of what sounds better since mousse is a relatively new invention.

Not sure that I agree with your reasoning or choice of words there, bel. I think mousse is an example of an uncountable mass of substance that Bingley referred to. I would say 1 moose, 2 moose, but never 1 mousse, 2 mousse. On the other hand, I might say "I had two different mousses to choose from this morning" which is probably a lazy abbreviation for "two different kinds of mousse". Same applies to a lot of uncountable mass nouns - try substituting honey, jam, oil, wheat, sand,.. for mousse (in the sentence, bel, not your hair!)