The slight variations in the covers come from different "printings" of the tapes, much like books have different printings. FHE started releasing the TMNT tapes with the "stylized" font, as you put it, then changed it to be more uniform so it'd look nice in box sets.
Pretty much all of the covers are available on ninjaturtles.com (a lot of them are actually scans of my tapes.. because Dan didn't have scans of hardly any of them and I got reeeeally bored one day
)