From personal experience I have not had any issues using third party PSUs in HP or IBM servers, if anything they were much more durable and in some cases more stable than the OEM supplies, the one issue I did have with an IBM eServer was the original PSU was slightly taller by 10mm so I had to block the gap off with some masking tape, all the screw mounting holes were in the standard placement.
as far as i know (i am not really sure) server psu's comes with ETX form factor instead of ATX. Like they are slimmer but longer. Electrically all psu's are the same. You wont find that much noticeable feature differencies. Mostly the quality and power range is different