PS1 games can't be played as an ISO, and have to
be converted.
POPS is the name of the ps1 emulator built in to the psp. A different version of POPS is in each firmware, and some user-converted games will have problems on new versions of POPS, which are loaded by the firmware by default. POPSLoader is a plugin that allows for you use older versions, usually something like 3.52 or 3.71, to fix problems that the occasional game may have on the newest version.
Here's a guide on installing it. While v3 works fine, there is an unofficial v4, I think it's mostly to fix problems with ME CFW rather than PRO.
Since you mentioned the go, that installation guide mentions there is no sound except if you use headphones when loading the only older versions of POPs that work. I don't know about you, but I generally prefer to play with sound, and don't like headphones.
If you're going to pirate them anyway, I recommend finding PS1 games taken directly from PSN, as they generally fix the problems that might arise from the user converted ones. They also come with better quality manuals, which are nice compared the scanned-in ones
here
http://wiki.gbatemp.net/wiki/Emulation_on_PSP
Here's a list of emulators. One that's not on there that I'd recommend, if you're into GBA games, is
tempgba4psp, but only for certain games that the other gba emulators have problems with, like Golden Sun and Mario Tennis, since it apparently it introduces new problems for other games.