I am a chemical engineer, not that I think this qualifies me to answer your question any more than anyone else. But...
Yeah, the old stuff will work just fine for chloramine removal. Perfect use for it. I don't know how many grams are in a tablet, but based on things I have read, you need roughly 1/8 tablet per 5-6 gallons of water.
Shoot... I bought my Campden tablets roughly 7 years ago, and I still have almost a full bottle and still use them today. I haven't had one single Band-Aid flavored beer since the day I started using them. It works like magic. And I swear the tablets will probably last like 50 years. I don't see what a little age is going to hurt.
My 2 cents.