For firmware choice, regarding ISO files, most custom firmwares play ISO files with nearly the same exact compatibility. The ONLY thing in the way, is that when new PSP games come out, Sony adds firmware updates on the UMD's. What that means, is that you'd need to update to that firmware in order to play the game.
*EXAMPLE* Keeping that in mind. If you had firmware 3.03 oe-c, and you tried running an ISO with a firmware update with 3.60 on it, you wouldn't be able to play that ISO.
Generally, it is recommended that you upgrade to the highest possible custom firmware in order to have the best compatibility with loading ISO files. Custom firmware 3.52 M33 and 3.71 M33 are good choices. Also, usually Sony makes minor tweaks to the PSP's internal Playstation 1 emulator when new firmware updates are released. Because of that, you may notice that compatibility with the Playstation 1 emulator is better with higher versions of custom firmware.
Also,
MEPHiST0, our tutorial section is a bit different than most other sites. We keep all of our tutorials in our "PSPBrew Master Guide to all Tutorials". This document has already been added to that list (in the second paragraph in the intro). If you still don't understand any aspect of the PSP, and can't find a tutorial for it in our tutorial list, I encourage you to make a recommendation that someone write a tutorial about your problem in our "Tutorial Request Thread". That way, if someone knows how to answer your problem, they may write a tutorial about how to fix it.
All the best.
-DarkPacMan77-