This can, technically - usually - be done ... however, you risk additional problems and possible data loss.  Drives and controllers have many many settings that are tweaked and set from the manufacturer, depending on their use.  OEM's (Dell, HP, IBM, etc.) have their own firmware put on the drives from the manufacturer that qualify/certify them for use on their controllers - to match the settings on the OEM controllers.  Depending on the settings, you may or may not experience issues, and they may or may not be major if you do experience issues, but the first thing you will look at if there is a problem is the non-certified drives in the mix.  Support is always free in the States/Canada for troubleshooting hardware issues, but if you are using non-certified drives (which are not supported by Dell), then one of the first things you will be asked to do to continue troubleshooting is remove any unsupported hardware.  If you ever expect to need help from Dell in troubleshooting issues with RAID, then I would avoid non-certified drives altogether. 10K vs 15K will have no impact on whether a drive will work or not ... they will mix just fine, as the controller will sort out each drive's capabilities individually.
Is there any problem with replacing failed dell 73G 10k ultra 320 drives in a raid 5 with another branded or oem drive. Am specifically looking at Seagate Cheetah 73.4G 15K ultra320. Not sure if there are any issues with mixing types in the same array. Thanks
Replacement drives for PE 2800