walleye...
those oddballs are all ambush predators, and all will get along fine w/ other ambush predators as long as they are all about equal size. So really any small lionfish, leaffish, scorpionfish are good tankmates. if you got the bucks, consider stuff like rhinopias, ambon lions
Croc fish (often called flatheads) are awesome when small, but can get around 2ft long (depends on the species).
What i find is that its been extremely difficult to get oddball fish, so you getting a popeyed seagoblin is an excellent start. i take it your feeding the tank live ghost shrimp as food