Seems bizarre to me that only six would fit. Especially since both single DIN and double DIN units will fit.
If you had your heart set on doing it yourself, I'd say to call them and ask why a certain unit does not fit. I checked the details on several and they all come up with the same chassis size for the unit, regardless of whether it "fits" or does not.
But, if you prefer to go with an installer I won't argue against you. If you're not comfortable with tearing the dash apart, that's perfectly reasonable.