Thanks BlackRevolution and shadow51981. Your posts cleared up a lot of the confusion.
And thanks Jack, the wiki helped a lot! I think i got the gist of spies now.
Just one question remains, When i activate a spy ability to hurt the mechs or take all infantry to 1 hit point, how long does that last? Should i wait until my attack force is one tick away before using these powers?
I have tried experimenting but my spies keep getting caught during the operations

Thanks everyone, you have all been a great help.