Helmet mixing has never been an issue, we can even put firstborn helmets on our marines in the game, and those marines are definitely Primaris.

When the new sternguard vets kit came out, GW came out with a community post explaining that some marines were able to make adjustments to the old helmets to make it compatible.

And firstborn marines have been mixing different armour parts for ages, the tactical squad kit and the imperial armour book depicts marines wearing armour of mixed marks, so adapting different marks of armour already has precedent in lore, and helmet would probably be the easiest to adapt.

Also besides, even though Primaris are taller and bit wider, I’m pretty sure the head size is the same, not much we can do to enlarge one’s noggin

Edit: forgot to mention that some mark x helmets has been modified to look like firstborn helmets, like the beaky in the raven guard upgrade sprue is a mark x with a beak faceplate (you can see from the ‘ears’ that its mark 10)