Yeah you can downgrade to 3.55 CFW since your base firmware is 2.70. Using this outdated multiMAN isn't really a problem since it hasn't really changed with the later versions and can still run games normally.
What, if any, limitations are there in having to use their specific 3.55 CFW vs. Rogero or Kmeaw?
I'd also like to use a newer Multiman since PSX support started on 02.07.00, so can it be 100% confirmed to work or not? Or only "Multiman no-check 2.05.03"?