Is there a large mushy section before it becomes firm?
Also, are you sure it's firm and you're not just hitting the floor?
Of course they're compatible, that's not your problem. Either you still have a s*** load of air in your lines or the master cylinder is bad.
have you tried to gravity bleed the system?
Also, you say you "almost" put a litre of fluid through the system, I usually go through 2 bottles when flushing the system; maybe I'm an over achiever?