Fast Track allows you to rank faster.
My question is, do i have to use the armor mod in a loadout all game to receive bonus XP?
No , but you gain more if you play the Full Game with it :
Example: Spartan Ops
Full Game with Fasttrack on Legendary Settings 9300 xp
Half Game with Fasttrack on Legendary : 8600
To recieve the maximum amount of bonus exp yes.
Though the boost in War Games isn’t that big of a difference, it’s Spartan OPs is where you get a huge exp bonus. 7k easy. Played Land Grab w/ Fast Track and got 8k :P.
to earn the extra xp you have to be using the loadout with the fast track armor mod. you can use other loadouts during the game but will only earn extra xp for the duration of time you used fast track.