Since the last update you can carry 2 guards in one march. The guardians have different benefits and must be chosen wisely to get the best performance for your march. If you want to collect iron, Ines Suarez at maximum level is best in combination with Voltaire. This will give you a 120% higher gathering speed. However, if you're going into battle, you'll need closer attention and choose.
Note: Individual Guardian benefits are activated after x rounds. The combat system is turn-based, which we do not see directly. So if you attack a player, x rounds can be calculated during the attack. Based on these rounds, the Guardian benefit is then activated.
gunsofglory.de provides you with this list, which you can download and use on your smartphone, tablet or PC. This is an overview of all guards, divided into colors and benefits.
Green refers to collecting, red to fighting and orange for beasts. Only General Dumas currently has an ability to gain more experience points for Guardians and skills.
So that you can read it here as well in tabular form: