Suigetsu can't even take on any of the Sannin by himself so way lower then a Kage
One of the Sannin held his own against the 6 paths of pain for a pretty long time.
One of the Sannin is an invincible tank who could outlast most opponents and leave the fight with no damage (keep in mind the only opponents we've seen Tsunade stand up against since part 1 are Madara and Pain, two god tier shinobi.)
And the last one is almost as invincible as Tsunade and could even survive some things she couldn't. Not to mention he has the power to command an army of undead and fought off a 4 tailed Kyuubi while in a weakened state. There's also a possibility that since his body is now composed completely of Zetsu cells that he could use the Dragon Sage Mode (it was explained that he had the knowledge of using Sage Mode Chakra as his cursed seals are based on it, but his body couldn't handle the sage mode transformation. So in theory with Zetsu's body he should be able to use dragon sage mode now, but that's all speculation).
My point is, Sannin level is at least equal to Kage level.
OT: Suigetsu would probably be considered Kage level, yes.