I think there is a difference between a mass shooting at a public place and a home style family shooting. Its not really analogous to compare the two.
I get your point, but don't agree and I don't agree because there is no definitional difference between the two and nobody seems to care about it when it comes up for discussion when they occur in gun-free zones. In fact, I would say that you are the first person I have seen suggest they should not be compared. They may not be analogous, but they are homologous.
Not only that, but most workplace mass shootings, while usually not family, do also involve the same concept of familiarity. The shooter knows most or all of his/her victims and often targets specific ones, sometimes to the point of being sure that others are allowed to leave safely.
Now if you wished to separate out mass shootings where the shooter did know his victims from shooters who did not, that might be an interesting distinction, but even in many of those cases, the shooter started out by killing people known to him/her first such as with Whitman and Cho.
If you think about it, you can come up with all sorts of distinctions.
Robbery mass shooting...
Family
Workplace (disgruntled employee)
School (student shooter)
Family workplace
Crazy person (e.g. Sylvia Seegrist)
Mall shooters
Maybe crazy (Carson City IHOP)
There are the shooters like Cho and Whitman who seem to be bent on killing all that they can kill before being stopped and those like Peter O. that have particular targets in mind and stop shooting after accomplishing their goal.
...and on and on and on. There are really a lot of ways you could break things down to separate out shootings and then say they aren't comparable because they don't share whatever distinction trait you think is important, but the bottom line of the mass shooting is that it involves 4 or more people and if you are in one, it doesn't matter if it is a family or non-family mass shooting, does it? At the root of it you are still left with the individual who is shooting multiple people and this happens in gun-free zones and non-gun-free zones.
So with that said, why would you say that they are not comparable and what does not being comparable mean regarding mass shootings? I disagree with you so far, but maybe only because I don't see the justification behind your statement. The statement itself doesn't shed light on a justification.