Sorry, I might've worded that poorly.
The feikorp rule applies when a model is hired as a mercenary via the mercenary rule, not just when it has the mercenary type. They can hire a feikorp guy and a ronin because neither of them are being hired as mercenaries.
Afaik, the only model that the Viks can hire with the mercenary rule is performers, which would prohibit them from hiring freikorp.