Tapajos may be too big for a 55, as while they are one of the smallest true geos they still need groups.Personally, I think the best options for a 55-gallon tank would be Convicts, EBAs, Geophagus Tapajos, or Angelfish. Some of these can mix, and others can't. Asfishhead0103666 said above, it depends on whether you want quiet, peaceful fish or aggressive "glass bangers."
Also, Striped Raphael catfish are one of the best options for a "mini-monster" catfish if you are interested in catfish. They are nearly identical to Irwini Soldier cats yet are a fraction of the size.