I had heard that he created his own lettering system so that his words would be easily distinguishable from the actual words of the Torah. Is this just hearsay or speculation? If so, is there any reason given for why?
According to Marvin Heller (See page 62 there), the printers used a different script to differentiate the commentaries from the main text. (Tosfos also uses "Rashi letters" as well as many other commentaries)