Is it okay to use standard weight J74 or J75 on a pretruss rod Flatiron A5. I would think so, since it’s a very solid design, but have not seen recommendations from Flatiron for using standard or lighter weight strings.
I have had my 1986 A5-JR strung with J74's, or other medium gauge strings, since it was brand new. That's what it came with. I tried light gauge several times over the years but wasn't happy with the sound or the feel. I don't know for sure, but it may have a non-adjustable neck reinforcement of some type. I never gave it much thought.
I used J74's and J75's on my 1985 A5-1 without issue. Flatiron pancakes however, shouldn't be strung with anything other than light gauge strings.