yes they will get along till your aro does not go on a killing spree, or if you are lucky then you will might end up getting a tame one. in that case they all will get along but i am not too sure about the featherfin
I cant say, depends on sveral factors, such as tanksize and the size of fish. Filteration etc.
Im asuming that the highfin shark is a pangasias species? I dont think it would go down the well, the syno should keep out of the way but could potentially become food for the arowana and pangasias and ive always found knifefish to be shy and reserved and i think it would get knocked about and might not get enough food.
Also that jardini arowana will turn agressive at about 12" in length and cause problems.
sorry to say but your gonna have to upgrade if you want to keep the aro ( chances r it will kill everything bit will still need a bigger tank), the knife and the shark, assuming it is a pangasius which will grow 4-8 feet depending on the species