It's not a lost cause ... not yet anyway
I've had both in the same tank before, and have to agree that the Sailfin was the most territorial.
Try an eggcrate divider for the tank (leave it in place for at least 2 weeks). That way the Sailfin can see the Foxface, while also having food arriving as before, with enough for both of them. One of the reasons for aggression is to defend the necessary territory to maintain an adequate food supply. Pod eaters seem to be notorious for battling in this regard.
With the fight for food eased, as long as the tank is big enough and aquascaped to give each their own space, you'll stand a good chance.
I've used this technique with an Emperor Angel and a Yellow Damsel on separate occasions, to good effect.
Good Luck
