You could add some keyhole cichlids or Laetacara.
I wouldn't add the severum because of the plants, and also because an adult severum would be a bit cramped in a 55 if it grows to its full potential. Severums are fine with rams, though.
I would avoid discus as they need to be in groups and a 55 would be a bit cramped with plants and other tankmates. Angels wouldn't be a bad choice, though. Unfortunately, I tried live plants once and my angels ripped them to shreds. They didn't eat them--I think they just had fun destroying them. I've seen a lot of planted tanks with angels so I'm sure that's not always the case.