“A rebbitzen should at least know enough to know whether a rav should be asked, so for tznius a woman should talk to a rebbitzen first.”
I am not sure that I agree with this statement. There is nothing wrong with a girl asking a sheilah to a Rav, and you can never assume that if you ask a Rebbetzin and she is not qualified to answer, she will tell you so.
I have had some bad experiences when I was forced to ask a Rebbetzin as a go-between to her husband (since she was screening his calls), and she thought that she was qualified to answer even though she wasn’t.
As one of my Rabbeim told his daughter who thought that she was qualified to answer sheilahs on the basis of being his daughter: “Would you ask a medical question to the doctor’s wife or daughter?”
Even if the term “Rebbetzin” is being used here to refer to a woman who is learned and wise (as opposed to meaning “the wife of a Rabbi”), I would still try to avoid asking questions to someone who is not qualified to answer.
Most people (even very wise ones) do try to answer questions posed to them even if they are not qualified to do so. This is either because it is human nature to want to help and/or to find it hard to admit that you can’t, or because they are assuming that if you are asking them, you have no one better to ask.
So it is YOUR responsibility to try (to the best of your ability) to figure out who the best person is to ask this particular question to before you ask. And there may be a certain amount of trial and error involved.