As far as what you can and cant keep him with, dont put him with anything that he can fit in his mouth because there is the possiblity that he will eat it. The 2 larger ones (around 8") that I have love silversides and prawn. If he is used to live food he might not accept this right away, but after 3-4 days of not eating they will switch over. Also try feeding right before the lights go out. As for my baby whos about 2" he loves black worms, and also eats shrimp pellets. You might have to place these foods in front of him if he is this size because he might not be able to eat before his tankmates do. With the black worms, just get a fork full and place them right in front of him and he should stuff himself. Also beware that these guys grow very fast and he will outgrow his tankmates quickly.
He should be ok in your 540 as long as it is at least 3' wide, also put a powerhead in there for him cause they love to swim in the current. Be carefull when you are cleaning the tank or moving other fish because they can be startled very easily and can injure themselves by hitting the glass.