It really all depends on the tank set up, size being very important and the individual temperament of the fish in question. Give it a go but never buy a fish to house with others unless you have the space in another tank to remove it if problems occur.
in my experience, aro and gar did not like each other. My aligator went through the separator just to kill of the silver. Both were puny juveniles then silver died shortly after. I did not house gar with aros since then (arapaima is not counted).
Gars and arapaimas: gar is a slow food taker from my observation but no aggression towards each other. Gars have been driven to the middle/bottom of the tank. Paimas rule the surface.
I have a jar but i would not mix that. My money is that gar will kill the jar in the juvi stage. Should jar survive then gar will be a very hungry fish after ...potentially dead.
As Ramesh said there is no harm in trying just keep an eye out AND be ready to separate/re-house .... after all this is partly why we are in this hobby to see what can be mixed and combined together.