Currently iOS Frotz only reads ZMachine and Glulx games, i.e., z1-8 (although a .z1 game might have to be renamed to .z3; Mr. Smith forgot to support the extension .z1 ), .zblorb, .ulx and .gblorb. The maintainer of iOS Frotz has mentioned that he does, at one point, intend to expand it to allow for TADS and possibly Magnetic Scrolls, but that’d be a ways in the future still, since he still has to iron out some kinks with Glulx support (namely, hyperlinks are somewhat broken and there is no music).
And as Laroquod said, I’m afraid the similarities in the extension are just that, superficial similarities. The .z80 file you have is probably a Spectrum game file, for which you’d need a Spectrum emulator. A .z1-8 file is a ZMachine file, which requires a ZMachine emulator/interpreter.
Now… there ARE posts out there about using the paid version of Spectaculator to play Spectrum game on iPad/iPhone/iPod Touch. I’m not sure they require a jailbroken device, maybe they just require a program that allows you access to the inner workings of the app, like DiskAid. You might wish to investigate that. Spectaculator Pro is by itself an app that comes with a few Spectrum games; you might be able to include your own Spectrum files into the app.