It is generally taken for granted that a similar rule or law must be in force for Federation civilians.
There’s been a lot of arguments on this point over the years but I stumbled across a pretty clear answer on this point.
BASHIR: You’re going to make a move against Koval, aren’t you? You’re going to find a way to use his illness to keep him off the Committee. I suppose it would be naive of me to point out that interfering in the internal affairs of a sovereign power is explicitly forbidden by the Federation charter.
http://www.chakoteya.net/DS9/542.htm
Interfering in the internal affairs of other species is, of course, the crux of the prime directive. So, yes, not only is it banned at the Federation rather than just the Starfleet level, it’s written into the Federation’s constitution itself.
I think the saucer separation ability was an example of theoretical thinking. The Galaxy class designers clearly intended a use for it. In dangerous times, the saucer would separate leaving the stardrive section free to act without it.
But, via practical real world experience, Starfleet learned that separating the saucer section was seldom worth it. It was too slow, where danger tends to occurred suddenly, and, even when there was extra time, Galaxy captains learnt the extra power generation of the saucer proved more useful than the decrease in mass. I doubt the War Galaxys from the Dominion War even had the ability and, if they did, it was only because it would take too much work to remove.
It’s possible that the saucer section worked better as a one-use ‘super-lifepod’ but the original plan of combining and transforming ships clearly didn’t pan out. Same thing seems to have happened with the Prometheus with Starfleet deciding the ‘wolf pack’ attack mode innovations, while effective, are better implemented via separate ships. Like we see a trio of Texas class ships doing to take down a Sovereign.
(There’s clearly some guy in the Starfleet bureau of ship design who keeps trying to make combining/separating ships happen and keeps getting disappointed when they don’t pan out in practice.)