Sorry I can't comment on your allergy question, as I am fortunate in that I do not suuffer from any at all.
I do though have Sjogrens and to get a definite diagnosis of this you must see your doctor. There are tests that they run to determine if in fact it is or not Sjogrens. Good luck with getting a good doctor to carry out this test.
Yes, I have costochondritis and have had it for a number of years. As I can't take NSAID's because they interact with a lot of my other medications, I have steroid injections into my rib cage for the pain and discomfort. This is only the treatment that was recommended for me though. Other people have different ways of treating it which are much less invasive.
Good luck in finding out if you have it. Again, it is only a doctor who can and should give you the diagnosis.