Fight, I think, might be an exaggeration if that happens.
Enne and Adori are literally strong because they have the blood of the FHs flowing in their veins.
Luslec and Baek Ryun, I'm confused as to why people think they are on a FH's level. Luslec climbed the tower as a servant of V. If he was on that level, he'd have received the same contracts as the FHs.
Baek Ryun seems to be talented from the blog info, but Gustang's failure to even acknowledge his existence as one of the Wolhaiksong is telling. He's just another of Urek's bug friends.
I don't know what can happen in the future but if at this point in the story, not one of those who speak about the FHs has ever suggested even one individual who could rival them, then it's unlikely such an individual exists among the currently known characters.
There's nothing about Adori being one of the 10 families, it's just a theory that by the way is already bankrupt because it doesn't make sense for SIU to hide it.
As for the rest, it has already been said that if Luslec appears, an FH will move, Lich said that Wolhaiksong has the power to go against Zahard's army, it wouldn't make sense for him to talk about the entire organization if it was just Urek's power.
Enne proved so strong that when he went mad, Zahard had to act directly.
Molic fought Zahard directly.
Not to mention there's the main point: the ratings, they are full of inconsistencies.
In the end, it's much more likely that in fact no non-irregular can have a fight with an FH, but going by Gustang's lines is a mistake. Both he and Trau were blindly arrogant. Trau says there's nothing he can't control in the tower, and we have Yasratcha, Nen Nen, WW, and Yama.
In summary, as long as SIU doesn't put an end to it and give us the reason why these 5 people are ahead of some FH in the ranking, there will always be room for this discussion.