I am considering a used Sony F3 and I am initially thinking of using Samyang primes with it. However for some jobs I would want to pick up the servo zoom for it as well. I am aware of the limitations of this lens, but my concern is this. If I decide to switch from the servo zoom lens and use a prime lens for an interview or specialty shot. Do you have to set the back focus every time as you would with an ENG- lens? Or is it a matter of switching from one lens to another? There is no place for me to check this out so any help appreciated.

If switching between different zoom lenses you might need to worry about back focus distances being slightly different for the lenses if they are not calibrated properly Ideally (at least with PL mount zooms) everything should be collimated properly - the same as being used on a 35mm film camera which has no back focus adjustment - and thus if the camera is set up properly all the lenses should be properly set for back focus.
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On prime lenses, the only real back focus problems arise if you can't focus to infinity, or if you need to use the exact distances on the lens footage scale. If you eye focus the second problem is not an issue. You are more likely to experience the infinity focus problem if using an adapter that is not the correct flange focal distance.

So my answer is -- you shouldn't need to adjust the camera's back focus once it has been set properly, unles a lens or adapter is not collimated properly.

If you're talking about the Sony servo zoom then the lens itself adjusts it's back focus internally and electronically. Back focus effects not only infinity but your focus witness marks will be out if the back focus is out. Also some retro-focal lenses (normally wide angle lenses) and internal focussing lenses may not focus as well as they should if the back focus is off. Provided no one has messed about with the cameras mechanical back focus adjuster you should not have any issues provided you use a well made and correctly spaced lens adapter.

Dave and Alister are both right , but I would add a few m,ore issues to be aware of.
If you plan to use par focal zoom lenses - either PL lenses or say Nikon or other still zoom lenses they will function like the ENG lenses you are apparently used to . Parfocal of course means it is a lens that is designed to hold focus throughout the zoom range even when the f stop is wide open. If the back focus is off then the lens focus will drift as you zoom.
In theory all lenses if properly set-up will back-focus properly if the back focus adjustment on the camera is correct. However theory doesn't always match practice.
A local rental house with very good technicians told me that they have all their PL zooms marked for which cameras they seem to work on correctly and that they have to adjust the cameras often when they go out.
If you use still lenses like Nikon or Canon you have the added problem of whether the adapters are perfectly adjusted and my experience is a resounding sometimes! No matter what the manufacturer says the tolerances may not be correct and have tried 2 expensive duplicate Nikon adapters that behaved differently

I own a number of Nikon zooms all of which seem to be pretty much par focal and an Optitek adapter. I had to work quite hard screwing with the back focus of the the camera to get the back focus right on the Nikons and once I got one they were all accurate. However when I put the Sony PL adapter on my camera and tested a few expensive PL zooms - some were perfect and some were off. I don't know to this day my F3 is set correctly for PL or whether I cheated it for the Nikon adapter.. So test test test I'm afraid.

One other thought on the back focus question.
Be aware that its a real PITA to adjust back focus on the F3. Its great that the adjustment is available at all and that its easily accessible from the front of the camera with an an Allen wrench , but there are no markings to see how far you've turned the adjustment and thus no way to set marks for different lens. I just found it confusing to know how far I had turned it any one direction or how far to turn it back again to get back where I started if I was going the wrong way. Nothing like how easy it is to set on an ENG lens.