Pterodoras granulosus also called the Granulated catfish, grow to about 3ft. So yes, a 600 gallon depending on the dimensions should be fine for them. They are not the 'monster predator' but yes will eat small fish.
For their diet they are very flexible, and will accept earthworms, prawns, mussels, catfish pellets, and their favorite ... snails. They may even take fruit, because i've heard in their natural habitat they do eat fruit.
If you have a Granulated catfish then bogwood or somethign which can give an area of hiding should be definately put in the tank. These fish at times are shy.
Granulated catfish should be kept with other 'large fish.' They do fine in water temp ranging from 70-75. However, avoid the warmer side as skins and eyes are sensitive to warmer water. This fish, at least of what I know, has never been bred in an aquarium, probaly due to the size of the tank.
These fish have a great personality. At times they'll become shy, then at times they'll be extremely active. When you feed them snails, you can see how much personality they hold. These fish are really great, but make sure if you do get them, they are atleast 6-7" because if they are small and your other fish are large, they wont be able to defend themselves.
They can be kept in groups. These fish are also nocturnal, or atleast eat in the night.
Just remember, if you are getting these fish make sure you can do weekly water changes, provide them with lots of hiding space, and if possible try to have a sand substrate.
If you do decide to get these fish good luck
BFL