But for the space marines they need to be able to put on their helmets and have them sealed,,, so they cant have any gaps at all,,, But i gues its just the standard warhammer 40k reasons,,, the rule of cool

Yeap, all hair must be sealed inside the helm, hermeticaly, after the helm is on.

This is why you play salamanders btw .. no animeboy haircuts just bald or supershort.

or you know, only wear helmet when its a absolute necessity

Stuff the hair into the seal of the undersuit. Helm will form seal outside. Don't go full Squat with your head or beard.

Spat my tea out, that was too good, oh my god.

I'll stick with Rule of cool was used in this situation.
Imagine dreadlocks, which there is some Space Marines that has. Do they need to grow new ones everytime they take their helmet on?

I agree with it being tied back and then tucked into the armor, as I have seen in most of the chest plates there is always a bit of room around the neck area. Of course if the hair is really long then it probably will still cause problems.

Salamanders have a special implant that lets them regrow their dreadlocks each time they take their helmets off

I would say one of three things happens. They either go in with no helmet on just like Space Wolves. They cram all that ish in their helmet and call it good, or the helmets are a lot bigger then we might imagine.

As for a more technical solution I would imagine the helmet has a kind of inflatable padding. This means that the helmet would have plenty of room for all your manly awsomeness and then the padding would inflate once put on ensuring a snug and protective fit!