The general rule is soft pattern, hard mould. Hard pattern, soft mould.
I tried several times to use PoP for hard foam castings, it was a disaster!
The best mould material for alumilite is probably RTV. But you are concerned about the expense. I suggest you try the PoP, but don't use your valuable, irreplacable master for the test.
Make a temporary master, but prepare it as you would your regular master. It must be sealed and finished smooth. It must be none porous, so the PoP has nothing to grip on to, maybe a coat of epoxy or something similar.
Do several tests, removing the master at different time periods. Try after 30min, 2 hours and over night. Removing the master early is the easiest, but you run the risk of damaging the surfaces of the casting as the plaster is still very soft.
If you are casting hard in hard, it is vital that the master is not die locked, in other words, you must be able to pull the master straight out of the mould with no twisting. For the same reason, deep flat sides should be avoided, too much surface friction, stopping the master withdrawal.
Search for information on sealing your PoP moulds, there is a ton of it, also search release agents. Without a release agent, you might as well save yourself some time and throw the PoP powder straight into the bin.
Best advice? Stick with the RTV.