LOL I am just dumb and been off the site too long. Solved and not a bug!

Once you select the pet slot totem, you have to activate the totem itself to unlock the actual pet slot.

There's no cap at all for loyalty box pet slots, but the caps for pet slots obtained from other places is on the user dashboard and the limits are 25 total from the Millionaire Center (at varying prices) and 65 total from the Cash Shop (unless you're already over the "soft max" on the first 10 of those)

Ah ok! I'm just dumb then lol. Forgot about the second part. thanks! also good to know about the limits on the mc and cs slots, I wasn't around when that was laid out :)