Calle,
No problem, happy to help any way I can. As for using a PC PSU, I actually got the idea from several RPi Clusters that I have seen built, like in the video below.
https://www.youtube.com/watch?v=i_r3z1jYHAcSeeing as I only have the initial Zedboard-based prototype and a Gen 1.1 Parallella board, as part of the EARLY-ACCESS pledge level, I really don't have much of a need yet for a PC PSU to power everything. Originally, I had planned on buying either a mini-cluster (4 boards) or cluster (8 boards), but upgraded my pledge shortly before the end of the KS campaign. Once they start accepting new orders, I will probably order at least a few more boards and build a small cluster.
As for powering a cluster of Parallella boards using a PC PSU, you could either connect the 5V rail to the mounting hole pads, like 9600 suggested, and power all the boards by connecting together with metal standoffs or build/buy several molex to barrel plug adapters and only power the boards via the barrel jacks.