It depends on the period you are talking about, Rome as an entity existed for approximately 2 thousand years, in various situations foreign soldiers vary between opportunistic and extremely loyal, such as the Gauls serving Caesar in his campaigns and in the civil war and the Varangian guard in the Byzantine Empire.And there were the Entruscans and Germans who, at the first opportunity, usually betrayed their Roman allies. At the end of the western Rome empire, foreign soldiers were one of the factors in its downfall.