I don't know anyhing about Minipress for nightmares, other than what I've read on the internet.
I use zolpidem tartrate (Ambien) for sleep. As well as aiding my sleep, it also stops me from having nightmares/dreams. I think this is because it reduces the amount of REM sleep. On the downside - when I try to sleep without zolpidem I have really vivid nightmares. I think they are worse because my brain is trying to catch up with REM sleep, but I don't know whether there is any scientific proof for this.