When the Goths became Roman Mercenaries, who were they tasked to fight, if not the Huns?
It is possible that some Goths became Roman mercenaries as early as the reign of Galerius (305-311 AD). See
Michael Kulikowski, Rome's Gothic Wars: From the Third Century to Alaric:
"Goths may have been recruited into the imperial army and served with Galerius in Persia, though the only evidence comes from Jordanes and is therefore suspect."
This did not prevent Roman-Gothic wars from resuming soon afterward (during the reign of Constantin, see the same book).
These wars continued throughout 4th century AD. Nevertheless, according to this wikipedia article, "As the Goths increasingly became soldiers in the Roman armies in the 4th Century AD, contributing to the almost complete Germanization of the Roman Army by that time, the Gothic penchant for wearing skins became fashion in Constantinople, which was heavily denounced by conservatives."
Alaric's brother, Ataulf, was hired by Honorius to fight first, an usurping emperor, then the Vandals and Alans that had settled in Spain. This seems to have been fairly typical.
The western barbarians often seemed to want to exchange their military service for Roman grants and honours, not imagining they could take down an empire that had lasted for many centuries, so many of them happily hired on to fight germanic aggressors not far different from themselves.