I went through this exact same thing with Taz. I debated for years. But the instant I saw Paizly here on Pet Talk I knew I had to have her. I did feel bad for Taz, he was also 13 at the time. But he took it pretty well. He didn't bond with her, but he just dealt with her presence. He hissed and growled at her sometimes, but I did catch him playing with her once or twice.
I am also going through a similar situation with Paizly and Jax. Except Paizly is only 2 years old. She's doing the same thing Taz did to her. She plays with Jax a LOT more than Taz played with her (but then Taz was sick). Again, I stressed out (I posted it here) about getting another cat. But I could tell Paizly was not stressed about him being here. Sure she gets pissy with him, but she is calm overall.

You never know, since Hobbes is gone, maybe Calvin would like a friend. Just consider what your options are if you find out it does stress Calvin out and you just can't keep the newbie. I knew I could take Jax back to ARF if it didn't work. As much as it would have killed me (thank goodness I didn't have to).
Perhaps two youngsters would be a better idea. Like Alani & Blaze for Tracey. They keep each other occupied so the elder kitty can be left alone and join in only when they want to.

I'm sure my post didn't help you at all, sorry. But good luck and just do what feels right to you.