I would just set the delays, or put 5 supplies in the case, it's really not a hard decision. I used to run a 32 processor cluster machine with over 4 terabytes of storage, and I can tell you now that I was never concerned about the time it took too boot up.
First, to keep the power supplies happy in the 2 2-terabyte drive arrays, the drives had spinup delays set on them.
Second, in order to power up all the nodes without tripping the UPSs which were running close to max load I had to start them up in groups of four.
If you want to build big, expect there to be some sacrifices in time to boot up. Personally I don't pitty you that you have to wait 5 minutes for your 1 terabyte worth of hard drive space to come online hehe. :roll:
One other alternative is to buy a large commercial switchmode power supply that supplies +5, +12. However you will find that a $80 PC power supply can deliver as much clean power as some $400 commercial supplies. I have found 100 Amp 5 volt supplies and 40 amp 12 volt supplies in dumpsters before.