Greetings,
The Phelps heads will fit on a Star, although a sloppy fit.
The Star heads will likely not fit on a Phelps. The hole is too small.
Phelps shell plates will fit on a Star, again, a sloppy fit.
Fairly certain Star plates will not fit onto a Phelps.
Might put the Phelps on eBay and stick with all Star machines.
Cheers,
Dave