I would put 2 or three needle nose gar and one elephant nose fish
for a 55g you should really have only one oscar not two never mind jack, get a bigger tank! but it is possible to house them together but the jack might end up kicking ass. and this will happen in a 55g for sure, you will get away with it for longer in a larger tank, if you are sticking with the 55g you would be best not getting the jack, best tank makes for them would be tinfoils or silver dollars but then they would need a larger tank but you should get away with it if you over filter.. min for two oscars should be 125g
The minimum tank size for one oscar is usually 75 gallons and your 55 gallon will become crowded pretty fast due to how fast oscars grow and i would suggest not keeping the jack in there due to aggresion that may become a problem.