Honestly, most native speakers would just use the English word
I've never spoken about spies in Gaelic and I don't really know... there's "spiothair" sure enough, but it's a simple English loanword. I would understand "fear-foille" as a deceitful person, a traitor or a fraud, so that's maybe not what you want, depending on the context... do you mean "spy" in the sense of a secret service agent or something?
By the way I think it doesn't really make sense if questions here are answered by people who don't really have much Gaelic themselves...
About the "your" thing, it depends of course on whether you mean singular or plural, singular is "do" and plural is "(bh)ur". This seems to be one of the things many English native speakers never get used to, that there is actually a difference between the two - not just in Gaelic, but in many languages
In many cases though, you can't use the possessive pronouns at all. So it would be useful to know a bit more about the context here, if there is any.