I found plaster to be OK to form over if there wasn't too much detail where the plastic will lock onto and sometimes chip the mould on release.I've heard of coating the mould with talc to aid release but haven't tried it persoanlly.
I now have fibreglass moulds and they are perfect.I can knock them about without having to worry about breaking them.If they did manage to chip,a little bondo/filler will do the trick.
I don't think you will have to worry about moulds withstanding too much pressure on a home made former,only pro machines will break your moulds (I think).You can always reinforce your moulds if you are concerned about them breaking.
Hope this helps,
-Paul.