You could try a smaller cichlid like a JD, Severum, Con, etc... However, they may not tolerate each other. A full grown Gold Saum should probably have a 55 to himself though.
Yeah you could add a rubber nose or bushy nose pleco, or two. I have heard that they like to latch onto Bichirs for their slime coat, so you would just have to keep an eye on that.
a small school of smaller, less nippy barbs or giant danios? I probably wouldn't add any more cichlids, except maybe another, smaller Aequidans sp. or something like that
I would suggest maybe another saum (opposite sex of what you have now) or perhaps a convict. My saum loves to chase my convicts and I have heard other people say the same. Just put one though. A Jack Dempsey would work too.
This is if you feel you must put something in there, of course, as the people who have said don't add anything else have a point as well.