Well if the Fex and the Qunari are from the same homeland, we can assume one of a couple things, knowing that the Qunari are something of a militant, expansionist people:
A) The Qunari have conquered the Fex, making them followers of the Qun.

The Qunari have some sort of peace with the Fex. This could either be willingly by the Qunari, seeing as the Fex give their society some benefit and aren't worth conquered. It could be an uneasy peace because the Fex and Qunari are approximately military equals and see no benefit to a war of atrition. Or it could be that the Fex are the greater military power and the Qunari maintain peace so that *they* are not conquered.
C) The Qunari and Fex are currently at war. Either that battle is something of a stalemate or the Qunari are actually losing that fight, hence why they seek to expand to other realms.
Other things that we can intimate:
Since the Fex are not seen often outside of Par Vollen, that could mean that either A) They are not a technologically advanced race and cannot easily leave the shores of their island home or

They are isolationists and choose not to leave and interact with the other races.
Other possibilities might include that their holdings on Par Vollen are not their homeland but a colony and that they are a race whose homeland is on another continent across the sea from Thedas.
We can speculate all we want about their appearance, but that's just shooting in the dark. They might be human, humanoid, mammalian, reptilian, amphibian, fish-like, avian, amorphous, whatever. Talk about shooting in the dark. But even just from knowing about the Qunari and knowing that this race somehow coexists in the same land as the Qunari, we can narrow down things about their culture.