How about a Wolfish? Would one get along with the Stingray's??? I know the Loaches and Electric Blue Cichlid would be in trouble once one would get big enough.
Or maybe...
Barracuda's
Payara's
Peacock Bass (I know it's not a Characin but they are nice fish)
lol even though i know little about them.
id put a knife. or Senegal.
if you can get payara then go for it. but everything except the ray would be in danger.
wait, i see a gar in the corner... hmmm...
id go bichir.
peacock bass will outgrow those in your list as well as the loaches and eat them.
barracudas grow slow...rays might try to eat em.
also...gar no go with cichla.
payara would depend on the species...