Interesting read, thanks.
Some of the stories and background fluff implies that indeed personnel can have different clearance levels for different SCPs.
I remember one (involving Dr Clef or Dr Bright maybe?) where O-5 offer him as an incentive "level 4 clearance on all SCPs" - which obviously implies that level 4 clearance can be (or is normally) limited to only certain SCPs.
In my opinion this makes perfect sense for Foundation as a way to limit risk and the potential for catastrophic security failures.
Very few people are going to need to know everything about every SCP. That's kind of what Oversights job is in the first place.