OP, I agree with the other folks here saying that you should start small when it comes to dosing clonazepam if you are not experienced with it. A half of one of those pills (.5 mg) under your tongue should help the anxiety pretty quickly, the onset will be faster than taking it orally. That's what I do if I'm having immediate anxiety, sublingual benzos take effect quickly and effectively.
Now I have no idea about allergic reactions to clonazepam, I have never met anyone who has had any adverse allergic reaction to the drug. If you take a small dose I would assume if you somehow were allergic, the reaction would not be as serious. But I am no doctor and don't know, if anything goes wrong a trip to a walk in clinic or other medical center couldn't hurt.