you are fine to install priiloader.
you can see, all slot number below 200 are either "no patches" (good original IOS), stub (officially canceled by nintendo, dummy system file doing nothing), or USB2.0 (IOS 58 from nintendo is officially supporting USB2.0, no patch needed).
All slots above 200 are custom system files, or action taken from nintendo to prevent people from using them by replacing with stub version.
222/223 : stub (piracy prevention, does nothing, nintendo expected to replace previous patched IOS files located here with empty useless data)
249/250 : the actual cIOS used by USBLoaders to play wii games from USB.
254 : bootmii slot, here stub by nintendo.
BC (Backward Compatible) : the system file to put the console in gamecube mode (official)
MIOS : the system file used by gamecube games (official)
I would recommend to install bootmii as IOS, as it can help you one day. you can run hackmii installer from HBC, then select bootmii as IOS.
Bootmii is used to make NAND dump full backup/restoration of the console's memory. in case of brick you can recover your console by rewriting the backup, or using keys to generate a new clean one.
bootmii can also be used to launch neek (an emuNAND feature). If you don't know or understand what it is, don't worry about it.
edit : seeing your drive is from 2007, you should even be able to install bootmii in boot2 (instead of IOS), which is the best brick protection you can have.
if you don't have it yet, from hackmii installer choose both installation of bootmii @ boot2 and as IOS.