1)Your custom firmware does have a ps1 emulator built into the firmware. It's called pops.
Error 80020148 means that the PRX type is unsupported.
Do you understand that you can't just run your ps1 games as .iso files though, right? You must use a converter which converts your ISO's into an eboot.pbp
2)Put floder and eboot in this way.
example
X:/PSP/GAME/MEGAMAN/EBOOT.pbp
Choose a brief name around 8 characters to be sure.
3)Maybe you try to run a official PSN store Psx game. Usually they have lastest official firmware. (last is 6.20).
If psx game from PSN has 6.20 you can't run it on 5.50GEN D3.