Hmm

I remember reading the light novel and rimuru said diablo and him are on the same level or similar enough coz diablo is so strong. I don't remember clearly so take it with a grain of salt. sorry i guess that didn't help.

I've read that the demon lords are more powerful and they are higher in ranks as they are second to the true dragons - - according to their ranks (disaster, calamity, etc). Guy crimson is the first demon lord and he was once a primodial demon, he has two primodial demons serving him.