I don't really know were this belongs. But i figured it belonged here. But anyways. Lets begin...
Before the founding of this planet. Far away from the imperiums grasp. A lone barge that parted from a expedition to explore new systems, found a planet untouched by any empire around the system. Surprisingly something within the planets core prevented anyone from finding it on there radar's. Once they touched the surface. They were immediately met by the planets inhabitants. A savage breed of human. The planet's creatures and hostile has given them a savage status, and a sudden lust for retaliation. They began to attack with there primitive weapons and blunt objects. And they were met by lasguns and autoweapons.
After a few tussles making a war for the next month. Once it was over. The savages surrendered. After a few uneasy meetings with there "assumed" planetary chief's. They successfully captured the planet and began to make a colony from the dust. With the assumption that the ship was K.I.A after the colony barges from the group were destroyed by ork ships. And the concealment of there mysterious relic. They were a renegade controlled. planet. Broken off of the imperium by all ways.
After a good few years of building colony's, they named there planet. "Taiberius" now. They are divided. And the inhabitants were divided. The rich and the nobles stayed either in orbit or were in complexes guarded by heavy security. And the middle to low waged were placed in slums and destroyed sectors. Ravaged by revolution and constant invasions by orks, demons and dark eldar. Suddenly. Trade barges were dispatched to nearby planets and systems. Which sparked the 1st regiment of Taiberius. A ragtag group of savages and lower class hive city citizens. First it was nothing but autoguns, blunt weapons with barbed wire wrapped around it. and the clothes on there back. They were tasked with protecting the barges with there lives.
Soon after that. They met there soon to be dreaded enemy. The space wolves. They were greeted onto there ship and were recklessly slaughtered. Followed by one of there trader barges. A month later, after the failure of attempted trading. The space wolves sent over many space marines followed by several platoons of armour. They lost half of there controlled planet, and the entire 1st regiment. But that wasn't it. Right before a attack on the capital hive city. They were met by over a million bodies of the 2nd Rifles Of Taiberium. They managed to kill most of the space marines and three quarters of the armour. Through sheer masses of men and guerilla tactics. They won the war.
But at what cost?
Soon after that. There planet and the inhabitants were casted renegades and heretics by the high lords of terra. And there planet was destroyed by exterminatus. Now with only 1/4 of the planets soldiers and civilians. They became drifters. With a huge trading barge, they converted it into a barge with some living qualities. But living in those conditions made them savage. Almost ork like. Weapons of massed origins. (coming from mostly there dead enemies) was there go-to weapons. After the massed las weapons were lost due to there planet being lost. They have no plan of finding a new planet. They will find there enemy. Strong or weak. And kill them, or die trying.
Keeping some imperial guard ways (Since they were imperial guard origin) they use commissars. Keeping the cowardly in line. The ones deemed "unworthy" were executed. And they absolutely despise the use phyker's and other's that kept powers of destruction. These are a dying breed of human, they use massed ranks to overwhelm there enemies. But that tactic is utterly useless due to the fact that the frail warriors die to quickly. So basically every battle just weakens the army entirely. Making the Taiberius race die out. But they love to fight there foes that laid waste to there planet. And will try to damage them in any way possible.
Uniform
Tactics
So in conclusion. This is what i came up with for my guardsmen fluff. Savage in fluff. But polite and tactical in game. But i try my best to unleash an entire horde of guard at the enemy. But it usually doesn't end so well. I still have a bad taste in my mouth after a battle with blood angels...

I enjoy feedback. (good and bad) and ill try to improve often. But that's it for now