small tanks are definitely less forgiving of changes in water chemistry than larger tanks and driftwood 'can' effect that chemistry in that if may add tannins to your water. these in turn 'can' lower the ph and gH slightly. thus, you need to be more careful ... but you can certainly add wood to your tank.
i put a nice piece of malaysian driftwood in my 10g and didn't have any problems. just keep an eye on your pH and do water changes to remove tannins and bring your pH back up.