When they use xenos tech they still have the mechanicus replace as much as posible with human tech to keep it "pure"

There are a few reasons the Imperium uses a relative handful of technologies: they are trustworthy and proven. The methods used to make space marines en masse *work*. Messing around with those risks having space marines at all. A xeno implant might superficially improve a marine's abilities but later prove to ahev devastating and irreversible side effects. Never mind the risk of xeno influence and domination.

I was planning on doing something awfully similar to this in my campaign, but with much more controversy and traditional warhammer 40k fluff working around, so I think it's a great idea.

In terms of background, it's pretty **** heretical - but the Inquisition which is kinda in charge of what is defined as heresy is awfully good at turning a blind eye to heresy when it suits it (cough, where did the grey knights get the dreadknights, eh ordo malleus, cough).

So it really depends on how it's RPed - if your KT is mainly Black Templars and/or zealots they will cleanse the whole thing with fire, and the augment can be a end-mission/myth-arc boss. If your KT is slightly more sane/populated with Salamanders who can see potential benefits for mankind (my KT) have it as more of a moral debate - have the augment not obviously more than a black shield at first, but then have their status revealed at a crucial moment, once the bonds of brotherhood have been formed, and consider the impacts then.

Not advisable to tamper with the work of the emperor. By adding more organs/ implants, the notion is that the work of the emprah is not perfect and is ground for heresy. The ad mech may incorporate or upgrade Xenos tech into existing tech.

Genetically modifying Space Marines is on another different level though. You may procede with your idea but it seems non canon and non fluff. In fact the emperor's children tampered with the space marines. Under Fabius Bile, they created a pool of modified geneseed organs to implant into those of Fulgrim's followers, creating the first noise marines and a plethora of other mutants.