The rule of thumb I learned here on TU is hard master/soft mold, and soft master/hard mold.
Mold making silicone is the best thing to use with a hard master. But it is relatively expensive.
If you go on Larry Dahlberg's website he has a video on how to do it.
If you're cheap, like me, you can make a silicone mold for a hard master with the 100% silicone that comes in caulking gun tubes, and then make a dozen baits with hard plastisol to use as masters for your POP mold. This way takes a lot more time than with the mold making silicone. If I had money to spare, I'd use Larry's mold making silicone every time I needed a mold.
The 100% caulking gun silicone is hard enough to hold it's shape, but soft enough to let you use a hard master if it's well sealed and smooth, even with reliefs and undercuts.
That way, you don't need a lot of expensive silicone mold material. A tube of silicone is like $5+-. If you coat your master with a thin layer, applied with your finger so it gets into all the recesses and details, you can surround that with more silicone that has a couple of drops of water mixed in it, and the silicone will set much more quickly. When you're done, your master will be completely encased in silicone, and you'll have to cut it out once the silicone is set.
You should still wait 24 hours before you cut down to the master and remove it, but you'll get a good mold. I make an open top mold box when I do that, and can slip the mold back into the box once it's filled with plastisol to hold it's shape.
If you make them with hard plastisol, and coat them with spray PAM before you embed them in the POP, they should last a while.
And keep the silicone mold so you can make more soft masters, if you need more.