As far as I inderstand, arimanni are rather cognate with harjamans, 'army men'. Still, there is no mistake. If you need such an allusion, use Hairumans.I chose to use Hairumans since the latin term Arimannus is attested for the Lombard warrior class.
Sounds reasonable.Imo with a game intended for an audience familiar with English it's better to use this without the article, as far as unit names are concerned.