This is mostly covered here: Third-party SSD solutions in ProLiant Gen8 servers
PCIe should be fine, regardless of the model. You may have to adjust BIOS setting to accommodate (cooling, etc.). The system fans may spin at a higher rate.
SSDs in a disk form-factor may or may not work. You have the additional challenge of finding HP SmartDrive carrier trays for the disks. Prior to the Gen8 servers, you could get away with this, but now you will receive firmware mismatch errors on the drives, or a notice that the disk is not genuine.
Update: I've tested the OWC Mercury Accelsior PCIe SSD in G6, G7 and Gen8 HP ProLiant servers. See the result here.