Only skirmishers can march and shoot.
Stegadons and Bastiladons are not skirmishers, so they cannot march and shoot :-(
They can make a regular move and shoot, though. And the would get the normal -1 to shoot when they did. However, our skinks carry Javenlins which are quick to fire, so they do not get the "move and shoot" penalty!
To sum up,
Stegadon can move up to 6" and shoot with no -1 penalty for moving and shooting.
If Stegadon moves more than 6", the crew cannot shoot.
Bastiladon can move up to 4" and shoot with no -1 penalty for moving and shooting.
If Bastiladon moves more than 4", the crew cannot shoot.
Troglodon can move up to 7" and shoot its venom with no -1 penalty (it is also quick to fire!)
If Troglodon moves more than 7", it cannot shoot its venom.