Well Apothecaries are the ones who harvest gene-seed. You can't really have all marines capable of harvesting gene-seed as that would bend the fluff too much and add even more special snowflake.
Making squads with mixed gene-seed makes sense but it also makes it highly impractical for the Apothecaries dealing with the harvesting ("Oh nooo

, Brother XX just got disintegrated by that D-cannon, we might be down on some Primarch _?_ gene-seed! Don't worry, I'll know once I run some lab tests... well if I can find some scraps of that gene-seed!"

).
And so on. Well okay maybe they have a list but it's still just a huge headache. Even with a full chapter it's still messy (like my post).
Why not just make a chapter based on the Mandalorians since you seem to like them? (Don't worry, I'm a big mando fan myself). Just add in some similar traditions and symbolism and convert that stuff to the
40k universe.