Tiger Shovelnose, Red Tail Cat (very agressive very active), Polypterus(bichars), Sabre tooth tetras a.k.a. Sabre Tooth Characins (very rare, super cool eating methods) FW Baracuda, Gars....the possibilities are endless.
Personally if i had a tank that big i would put a shoal of about 20 piranha. You can do mixed Pygocentrus species...including Red Bellies, Piraya, Caribe(black heart). THat would be sweeeeeeeeeeet.
Good luck with whatever you wind up going with. I envy you. I myself have a 220g and that is enough work for me. I couldnt imagine a 600g. INSANE!