Is it always better to put legendary heroes as first in formations?

No. It depends on the skills of the heroes. You need to check what the range of the heroes are. Most often that is shown in the information about skill 2. If the hero has a range of two or three that means you can put them in the second or third slot and they will still be able to reach the opponents. If you put a lesser hero in slot two that only has a range of one, they will not be able to reach the opposing troops and will be ineffective. Instead of thinking the legendary hero needs to be number one, i think of having the legendary hero flanked with his lieutenants in slots one and three.