I didn't know the numbers of foreigners in the army, but I knew it got bigger and bigger. I think that the Praetorian Guard eventually become foreigners. Which seems like a strange idea, to trust strangers to protect your leader. Although the Swiss Guard guarding the popes has been highly successful.

VERY common.

The Varangian Guard (Vikings) were the elite bodyguard of the Byzantine Greek emperors.

The Janissaries (European Christians, Albanians, Hungarians, Caucasians, etc.) were the army of the Ottoman Empire.

The Hessians (Germans) fought many of the battles of the Revolutionary War for the British.