The Seguleh are a rather interesting culture. As you surmise, they are a warrior culture and are known throughout much of the Empire for this. However, they rarely leave their island - those that have done act as their 'army' in a way, as they deem only a few Seguleh warriors are necessary to carry out any punitive strike on another nation. Their entire society is ranked in order of combat prowess, with The First being the greatest Seguleh warrior on the island - until another bests them in a fight. They wear masks to hide their faces and rarely speak as they don't deem others of other nations, or those lower in their own society, as worthy of being spoken too.
I would say that Helicrates probably wouldn't fit quite into the concept of a Seguleh - they would never be a mercenary and they would never work with others. In the books, the only two that do are forced into servitude by a powerful ascendant.
What if Helicrates were born a Seguleh, but perhaps as a child is somehow taken from the island and raised amongst Darus or Napans or even Malazans - not quite raised to the Seguleh way of thinking but retaining their eagerness and aptitude for battle. The only problem with this is that on encountering other Seguleh, they would not consider Helicrates to be 'one of them' as he would be 'tainted' by other cultures and deemed less worthy than them.
One thing that occurred to me was that perhaps Helicrates came from the mercenary band of the Grey Swords - from the city of Elingarth Genebackis - these are a band of warriors sworn to the Fener, the Boar of Summer. Perhaps Helicrates was born in Elingarth and joined the Grey Swords but however he came to wear his cursed armour caused him to lose faith in Fener and strike out on his own as a mercenary? Just an idea!
If your heart is set on Seguleh, we'll have to go down the more 'dilute' route I've described above.
Hope that's helpful!