Would Youngster (Friendly TIE fighters at Range 1-3 may perform the action on your equipped EPT

Upgrade card.) be able to give Ruthlessness to all other Tie's or just one?

There is no "action" on the ruthlessness card (the same way that Expose has), so no, Youngster cannot spread ruthlessness to other ships.

For Youngster's ability to apply, his EPT has to have the Action: header.

Putting Ruthless on Youngster would allow it to apply to one TIE; that TIE being Youngster.