where can I find a link or a list of PSU that don't require minimum load on 3.3V & 5V rails? google didn't helped:/
Try looking for a PSU that draws 3.3V and 5V from 12V rails, usually they will be marketed as 12V only PSU, DC-DC PSU, also a lot of PSU with single 12V rail will be in that group. Also all of those PSUs will be more expensive, that those requiring load on 3.3V and 5V, so you may consider buying cheaper PSU and powering with it a couple of fans, or an ethernet switch.
I never heard of this. You have to draw power from these lines?
All cheaper or older PSUs require that, if you do drain any power from those lines, 12V line may be unstable. Since all computers require some power from 3.3V and 5V, until few years ago it wasn't considered a problem. If you want to know more you can google for "one computer two PSUs", or something similar.