This is a complicated answer, but I think it will suffice.
In order to throw a 6-12 ft arc for slow pitch, the speed of a pitching machine needs to be adjusted down to the slower side of things and the angle of the pitching machine head needs to be tilted upward, creating a looping arc of a pitch. A lot of trial and error needs to be done until you find that perfect speed/angle combination that causes a ball to arc a bit before crossing the plate.
Heater Sports says that this can be accomplished with a cage as short as 24 feet, however, they recommend removing the lower section from the tripod legs so that the pitching machine sits lower, allowing it to throw more of an arced pitch from that position.
Heater Sports says that you may also wish to purchase extra Middle Pole Assembly pieces (part #B-2) with their batting cage in order to raise the middle portion of the net higher than normal. This would allow you to achieve an actual 12 foot arc without hitting the top of the net. You can only order those extra middle section poles directly from Heater Sports and they cost $5-6 each. You can see what they mean if you look at the Xtender Cage assembly instructions.
I hope this helps!