I have some question about them, but I don`t think 2 questions worth creating a thread so let`s discuss them here.
1) Can only specific sub faction use them?
Fenris pattern power axe- space wolves
Golden fixa- bad moons
2) Did only I saw golden fixa in war on arkhona trailer?

To my knowledge they can be used by the whole faction, not just the specific sub-factions they come from.

The axe/wrench at the beginning when they first showed off the faction with splash art was not the Golden Fixa. At 1:23 it might've been the Golden Fixa, but it is hard to tell with everything being washed that kinda of grey-ish murky color.

There are items that are warband/chapter specific, like bolters or axes.
There are pre-heresy items in founders store and there may be some in game.
Rare items may be found in raids and dungeons, but there is still a bit of discussion about it.