Nothing significant has changed much in the way of CFW for quite a long time. Those "outdated guides" still work.
Multiman guide by Cyan, still works.
How to Play PS2 Games on PS3?, still works. It's one method of running PS2 Games.
My guide isn't even exactly a guide, it's an information thread mostly to educate beginners with the OFW section and a simplified installation guide. (As much as I could simplify it)
Mines is the least "outdated", I guess.
COBRA ODE is a different matter altogether.
As for your questions, you need a 3.55 firmware or lower PS3. No BUTS.
Anything above needs to be downgraded but only certain models who's original manufacture firmware was at 3.55 or under can be downgraded.
Downgrading is a hardware mod using a piece of hardware called a flasher and the use of soldering. It is a complicated process and I highly doubt you could perform it on first try. Hire another person to downgrade it for you
CFW to install: REBUG, no questions about it. It has COBRA USB support embedded in the CFW if you want to play PS2 ISOs and avoid all that PS2 Classics placeholder stuff.
Safest way? PSNPatch. It disables CFW stuff temporarily.
Still there's no foolproof way of avoiding bans. Just use common sense, don't go on popular servers like Call of Duty and GTA.