As I said, the guy who made Prometheus worked on PRO. The 6.x firmwares are a continuation of the 5.x plugins (a project to improve compatibility).
So you're asking if a later software by the same author with a later base OFW aimed at compatibility is a good move.
That's why I considered it a stupid question.
You can downgrade any PSP to a software it's already had or one between, so you'd be able to downgrade to 6.20 just fine.
The risk is with people trying to downgrade 3Ks to firmwares they never had (like a stock 6.60 machine downgrading to 6.20).
So this doesn't even apply to you.
That is, the force-downgrade risks you've probably heard about.
CIPL is the perm patch method for 6.60, which does not work on 3Ks. It uses the older method which only works on 1Ks and the older 2Ks.
The guide states that 6.20 is the permanent version because if the IPL on later 2Ks or 3Ks is modified, it will brick them permanently.