Striker Rhys joins on a one-year deal from Port Talbot for a first shot at the Football League at the age of 32 after years of leading the goalscoring charts in the semi-pro League of Wales.
Midfielder Johnny, 19, is at the other end of his career. He has played 10 games as a professional - nine of those matches full internationals for Northern Ireland.
He comes to Home Park on a youth loan until January 6 next year.
Rhys, who is putting on hold his career as a firefighter to become a full-time professional, has trained with Argyle this summer.
He also played in the pre-season friendly against Truro City last month, impressing with his robust centre-forward play and scoring the only goal in the Pilgrims' 1-0 win.
Rhys will wear the number 10 shirt, while Johnny has been given number 28.
For exclusive interviews with Rhys and Johnny, see www.pafc.co.uk later.