The Snug Ride car seats will harness to 22lbs or until there is less than one inch of hard shell above the childs head. Their legs hanging off the ends are not a problem what you need to pay attention to is the weight and less than 1 inch of hard shell above the head.
When they outgrow the infant seat, you purchase a convertible car seat. Aconvertible car seat will rear face to 35-45lbs (depending on model) and then forward face to 40-80lbs (again, depending on model). You want to keep your children rear facing as long as possible, hte AAP recommends children remain rear facing until at least 2 and 30lbs, or until the child outgrows the convertible car seat rear facing. Google EXTENDED REAR FACING for more information.
As for traveling, especially with small twins, bring the car seats on the plane with you and buckle the kids in just like in the car. It will make your life alot easier, trust me. Use the double stroller and get a cart to wheel the car seats.