You want aggressiveness behave or mini predator that able to look at other fishes for a meal? There is not much choices for 36G tank though.
A shoal of Exodon Paradoxus, Juvenile red wolf (Erythrinus erythrinus) but it will need bigger tank later and most likely will eat any fishes that would fit in it mounth).
U could keep a electric blue jack Dempsey in there for about 2 years before u would have to upgrade maybe even longer there a super beautiful fish probably even the the most beautiful colorful fresh water fish in my opinion when they reach full maturity theres a pic of one of mine as a baby in my avatar in a 30g my pair are now in a 75g but they stayed in that 30 g for a long time becuz they grow at about 1/2 to 2/3 the rate of a reg jack iv had mine for over a year and they have only grown about 4 inches or so.
Dwarf pike cichlids would fit the bill as would some of the live bearing pikes. There are a lot of nasty small fish that will work for you. There are admittedly fewer predatory fish that stay small.
What type of wolf fish if red or purple erythrinus maby if any of the other larger more aggressive wolf fish in the hopilas family than no they get pretty big and mean as well.