There is no direct evidence from the known literary source of Tacitus, which I'm sure your familiar with,which indicates the Harii as ecstatic warriors ,or even that they were other then just black-painted nocturnal fighters.
Kershaw suggests that there is an implication within the source that the Romans believed their appearance and strategy to be a form of psychological warfare, but there is conjointly the implication within the Tacitus passage that is a custom to blacken their appearance; which would infer that is was not used as an occasional ruse and begs the question would an enemy be fooled be such a tactic as a paint job and night attack more then once or twice? The proposed custom is said to representative of the the Harii, or Herr, the army of the Lugii perhaps as an 'army of the dead' which may ring true if an Herr was partly composed of the Mannerbunde, that is the youth of the tribe.
Simplistically, this mannerbunde model proposes that as a youth, they are said to be in a liminal state being neither boy or man and as an initiate the youth are separated from the women, and other non initiates and are said to be 'dead' and belonging to the honoured ancestors and it is this concept that is important to the Harii as a sacral and possibly ecstatic warrior.
The period of passage is spent in the wilds outside of society were the martial skills and learning tribal knowledge is undertaken under the tutelage and leadership of an older warrior, they dressed in pelts or perhaps even painted themselves 'black' representative of there status as 'the immortal dead', the dead are immune to fire or sword much alike the later Norse Ulfhednar and one can imagine them rushing wildly with enthusiasm into guerilla style combat battle with little restraint eager to show there bravery much as Tacitus describes.
Perhaps the 'mercenaries for hire' you mention derives from the belief in Tacitus that such young guns would look to other martial leaders as a source for succour and battle honours if their own leaders did not provide for those wants, but it is difficult to say with any certainty that they were wolf-warriors.
Speidal sees germanic wolf and bear warriors among the vangard of Trajan's troops during the Dacian Campaign depicted on the well known triumphal column, and if this is correct it would tally with the Norse Ulfhednar warriors of Harold Finehair that you recall.
There is also a historical literary source that describes Langobardic 'hound headed' warriors which would indicate wolf warriors.
It is likely that the Ulfhednar and Berserker of Norse tradition are the final echoes of these earlier warrior cults and the belief that again that the Ulfhednar were impervious to fire and iron would indicate an ecstatic state, and if seen as Odinn's men they are dedicated warriors, filled with the battle fury cognate with that god.
Berserker warriors appear in the sagas as literary motifs, becoming marginalised loutish, thuggish figures and this may possibly be a reflection of changes in belief.
For a discussion on the Berserkergang see the below which covers most of the later sources and signals of belief that can be sifted from them.
http://www.scribd.com/doc/3208892/Shapes...hen-Grundy
The above is just scraping the surface Dan, and there is much more to say but would take another book on the subject!
best
Dave