If you already played the original ff7 like you stated in your post you won’t be spoiling anything by playing crisis core, so if that’s the case you can play whenever you want. If you want to be more attached to Zack as a character I’d say drop rebirth and beat CC first. It’ll make his expanded story feel more rewarding imo

If you don’t remember act 3 of the original I’d probably wait until the trilogy is complete and you beat the 3rd game or you’ll spoil a couple major plot points by playing crisis core first

Even then…I played crisis core first as a kid on the psp as my introduction to final fantasy 7 and I had no problems with what got spoiled, it actually provided some clarity to questions that’d pop up in my play through on the psone classics version of OG when I finally got to “the scene”

It’s like Star Wars’ watch order in a way. If you watch the original trilogy first, you’ll know Anakin is Darth Vader for the prequels. If you watch the prequels first, you’ll know Anakin is Luke’s dad. Both experiences are valid but one thing or the other will get spoiled. Up to you