I think it feels a bit strange to make a civilization of mercenaries actually have the special ability to hire mercenaries- when in fact they are supposed to be the mercenaries themselves.
It would probably be better to attempt to reflect their sellsword nature in gameplay somehow, such as allowing them to join into ongoing wars between factions with very few, if any, diplomatic penalties and actually making other leaders pay them to fight on their side.
In addition, if the Hippus had joined into a war between other factions, supporting one side it would probably be interesting if they were paid for the units they killed, with substantial bounties paid for capturing cities, and that those cities they took over while flying the banner of the other faction was handed over to their employer.
In gameplay this means that the Hippus would be sticking their grubby little fingers into every little war imaginable and earn gold and experience as they undermine the war-effort of their employer's enemy, all whilst suffering very little diplomatic penalties and not being subject to whatever applicable sansaction there might be from other factions.
I believe that would be a much more interesting and flavourful approach than simply mercenaries hiring mercenaries. Everyone hires mercenaries- whereas mercenaries are unique because they sell themselves.