"How do i measure that??" You may ask, but it's actually pretty simple, are there any precedent for it in the lore? then it most likely is lore friendly.

Though there are of course things that are kinda on the edge

This will of course require some self reflection

Lets go with a scale from 1 to 5.

1: Straight up breaks the universe, this would be anything that directly contradicts fundamental laws, major historical events, or core faction traits of the setting.

2: Extreme Stretch, It could theoretically possible under unique circumstances but contradicts normal faction behavior.

3: Plausible, Fits within the vastness of the galaxy but relies heavily on rare exceptions or loopholes.

4: Lore-Accurate, Has perfect alignment with current codices, novels, and established universe behaviors.

5: 100% lore friendly, it integrates obscure lore fragments, handles nuance perfectly, and captures the bleak tone.

Note: Even if you are at 1, that is completely fine.

I would put my Chapter, the Swag bears, at worst, at 2, and at best, at 3 or maybe 3.5, The biggest problem is their name but i have explained that pretty well i like to believe, but there are also the fact they are a pre great rift Space Wolves successor, and they are very kind, and there is also the whole Ursinen thing, which are basically wulfen, but bear.

And again, i already mentioned it once, but i wanna be super clear, it's okay if your homebrew is not lore friendly, you can have a female space marine chapter, you can have a chapter that are best friends with xenos, you are allowed to have that.

My Chapter, the Swag Bears, accepted them with open arms.

They allowed them to prove themselves on the battlefield, and while they were impressed by their physical prowess, the Primaris were clearly lacking in experience, which isn't a big deal, that comes with time.

So Chapter master Goldrum made sure to make mixed squads of First borns and Primaris so the Primaris could learn from the First borns and intergrate into the chapter's culture easier.

Now there were still some in the chapter who were against the Primaris, or at least had their doubts at first, but since it has been over a century now, the Primaris has fully intergrated and most of the Chapter has crossed the rubicon.

My chapter, The Swag Bears, is very kind towards loyal imperial civilians, they will happily protect them and do things most chapters view as below them.

Like searching for lost toys in rubble, or give comfort, though due to their culture, they do not view child soldiers as wrong, since you are forced to mature quickly on their homeworld, and they will also pull someone crying during a battle on their feet and tell them fight, but again, that is due to their culture.

The best way to think of them is to imagine Salamander Space Wolves (I did actually have them as a chimera geneseed once, where they were a Salamander/Space wolf successor, but i changed that to be only Sons of Russ.)

My chapter, the Swag Bears (yes i am aware it's a silly name.), are mainly inspired by nordic culture, since i am from scandinavia, so i feel very close to nordic mythology, and i like to think i am pretty knowledgable when it comes to norse myth.