Is there any benfit to list the category on some second and third generation (1974-1993) Ford Mustangs as "Hatchback"? Most are already classified as coupe, which I would think is more appropriate, but there are a handful categorized as hatchbacks. http://www.imcdb.org/vehicles.php?resultsStyle=asImages&makeMatch=2&make=&modelMatch=- 1&model=mustang&modelInclModel=on&modelInclChassis=on&class10=10
Would it be correct to consider them more of a liftback or fastback even though the rear glass window does lift open with a function similar to hatchback?
There was a discussion here: http://www.imcdb.org/vehicle_4595-Ford-Mustang-1990.html

Doesn't hatchback imply a car with more of the shape like these with a more nearly vertical rear window? http://www.imcdb.org/vehicle_56587-Honda-Civic-1984.html or http://www.imcdb.org/vehicle_42646-Volkswagen-Golf-Typ-1J.html

Also, the 1982 Collector Edition Corvette and all C4 Corvettes have a rear glass hatch which opens also, but those should definitely be classified as coupes, right?
http://web-cars.com/corvette/1982-2.php (3rd picture down) and http://web-cars.com/corvette/1984-2.php

what is the actual difference between liftback and hatchback? and about coupés and hatchback i think we have a problem, when can we consider a car an actual coupé and when just a hatchback?

Lots of coupé-shaped hatchback are usually called and listed as coupé.
As Wampa-One said, usually the hatchback has a more vertical rear window. Those considered as coupés have a sportier appearance. See VW Scirocco vs Golf/Passat...
I do not know what is best for the Mustang, but I would agree with coupé.

See? but the problem is still there, that of "Those considered as coupés have a sportier appearance" is very relative... in some cases it is probabaly clear but its not always clear when a car has a sporty appearance or not...

It is not clear but that does not mean that we have to list them in both categories We have to make a choice.
Same for other "hybrid" cars like Chrysler PT Cruiser (wagon/hatchback) that we finally list as wagon, or many crossovers for which SUV, hatchback, wagon or MPV could fit.

I think on the Mustangs up to the mid-late 80's there should be since there were coupe Mustangs and there were fastback/hatchback Mustangs. In the 90's weren't they all just the fastback/hatchbacks.

Coupe: http://www.imcdb.org/vehicle_27164-Ford-Mustang-1987.html
Hatchback: http://www.imcdb.org/vehicle_21157-Ford-Mustang-1987.html

I'd list all Mustangs as coupes. That's obviously what they are, as opposed to compacts/superminis and non-sedan family/executive cars.

For me there's a problem, that on American films a lots of common two-door sedans are listed as coupés, such as Saab 900, Toyota Corolla, Datsun 210 and such.
For me, the coupé is the car with low, sporty roof, with small +2 rear seat, or no rear seats at all. But these cars, like the 900, have normal roof and room in the rear seat for three people.

Indeed a coupé is supposely a 2-seater, or 2+2 at most (or 3-seater, like the Matra Bagheera & Murena )
Usually I reclass the SAABs to hatchback when I see one as coupé, but I did not made that systematically for all.
But some 4-seater hatchback are really considered as coupé rather than hatchback, see for example the VW Scirocco (hatchback being the Golf) or the Renault 15/17. But those have a more "sporty roof" than the SAAB 900 maybe, so it would be compatible with what you mention? (despite the number of seats)

17 and 15 clearly coupés, so is Scirocco.
But Ford Falcon is not.
http://www.imcdb.org/vehicle_60944-Ford-Falcon.html
A Coupé Falcon is called Mustang

I agree that this is a 2-door sedan, not a coupé.

Since long time I wanted to make a post on the forum explaining what to put in each category, maybe it would be the time to do it then.
In the same way, a better definition of the "supermini" category would be useful. I usually put in that the cars that are less than 3.5m length, bigger ones going in sedan/coupé/hatchback categories.