Gonna have to agree, it's probably best to let him finish his life as a solo fish. A 55g is not really big enough to accommodate two large fish, however... If you are simply just bored with a single fish there are other options besides adding another larger fish. (ie. catfish/pleco)
"Target" fish are great provided he can't catch/eat them.
I've recommended them before and also curently have them in my tanks as well. Giant danios are great to help add more movement to the tank. This however might not always work, my Trimac didn't even second look at them, my RD started eating them, luckily the bichir is to slow haha.
Lastly if you were gonna add any more fish I would also recommend bumping up the filtration.
Oh and great looking fish. =)