I have a PS3 Slim from the god of war/Kill Zone bundle and i want to put custom firmware on it.

its on OFW 4.55 or whatever the one from the start of this month was ( updated it to grab a game from PSN unfortunately)

from what i am told it needs to be downgraded, I also want to buy a bigger hard drive to put in it (1-2 TB) would this need to be done before i mod it or is after doable?

I also have a PS3 Fat thats backwards compatible ( I believe its an 80GB) If my Slim isnt moddable then this will have to do (i prefer the slim however) as well as the same question of would the hard drive need to be upgraded before dowgrade/mod

and yes, I tried google but apparently i'm either blind or google illiterate because i cant find the answers