Well it could go either way really as there is already precedent in the game for two different concepts of using magitek armour in battle - after all, in the Praetorium, you have to use the magitek armour to smash through barricades, thus the player is given extra abilities while riding it (utilizing the magitek armour's weapon systems), so then it shouldn't be too hard to extend those abilities into the actual game (and other mounts also have similar 'battle' abilities - i.e. the beastmen mounts like the laurel goobbue which are used in certain beastmen dailies), and allow players to fight while mounted, FF Tactics style.
And also don't forget that even though magitek armour is normally just a mech, a walking car in a sense, the player's suit of magitek armour
actually is sentient, given the fact in the storyline it was repaired with a mamett heart, effectively a magical computer - it was even christened 'Maggie' in the storyline as a token of affection - it even saves the player at the end of the game after destroying Ultima Weapon and defeating Lahabrea, when it jumps down through the flames to the player on it's own accord,
so having it fight alongside the player like their chocobo companion is definitely possible narrative wise. Either way, I'm really excited about this, being the FFVI fan I am.