Would these two get along in a tank together? Bear in mind that there would be literally no other fish, and the tank size would be around fifty gallons, with appropriate settings for each animal.
I don't see any problems with it, though I don't have much experience about arrow crabs but to my knowledge it should be harmless to the hawkish. But yeah you can def add more fish in a tank that size. Just make sure to go really really slow. don't rush anything!
I know I could, these are just something I've had in the back of my mind for a long time now and it would be a personal goal completed to start a tank with just them for awhile.