I generally try to keep my PRI's in one cabinet just so I don't have to use the clocking cables. As far as using cabinet 0 and 1 only, there is a diagram in the install manual that shows a PRI in cab 2 under how to run clocking cables. I think they might be concerned that clocking will not carry through the SCSI connection to cabinets 3-5.

The only restriction that I know as far as card placement in a particular cabinet is with the VIOP cards.