There is no formal guidance (that I'm aware of) about revealing a secret, top secret, Q, or L clearances, nor is there formal guidance on revealing that you are SCI eligible.
Each SCI compartment and SAP has its own unique rules about what you can and cannot reveal (some SCI compartments mere existence is classified at the SCI level itself, meaning you can't say you hold that clearance to anyone who doesn't hold that clearance, for example).
Each SCI compartment also comes with it's own unique rules about when you can or cannot talk about what you learned or were cleared to access (most are probably lifetime NDAs).
That said, it's pretty shitty opsec to tell someone, or the Interwebs, that you hold a specific clearance or do a specific job.
I'm not aware of one. In some cases, the information will just be disclosed, and you'll be forced to sign the paperwork and be 'read in' after the fact. As long as you have a legitimate need to know, that's fine.
Each SCI compartment and SAP has its own unique rules about what you can and cannot reveal (some SCI compartments mere existence is classified at the SCI level itself, meaning you can't say you hold that clearance to anyone who doesn't hold that clearance, for example).
Each SCI compartment also comes with it's own unique rules about when you can or cannot talk about what you learned or were cleared to access (most are probably lifetime NDAs).
That said, it's pretty shitty opsec to tell someone, or the Interwebs, that you hold a specific clearance or do a specific job.