a good strategy to find the fuel tank combo code is to change the red one, then run through all 4 options on the blue one. if it doesn't say 'success' then change the red tank to the next icon, then go back and cycle the blue one through all 4 options again. keep doing this, and you'll get it in no time.
Lando's an odd duck.. who knows what his bag is. If you kill everybody, then go up in his ship, he should follow you up in there. Watch out though, because as soon as you succeed with the fuel pumps, a weequay will have snuck up into the ship and be blasting at Billy Dee. You don't have long to get back and cut him in half, as Lando has apparently misplaced his blaster..