Oh, do you go to XYZ college? I too went there 30 years ago! Then you'll be my 後輩. I'm happy to see you. (What would you say 後輩 in English here?) 听起来自然吗？

That sounds natural, except you don't need to include the word "too" in the first sentence. Since you are telling the person that you attended the same college, that word doesn't add anything else to the sentence.

About the word 後輩, we don't really have a word that we use in the same way. I think that part should just be left out, because English doesn't have the same system of honorifics.