There’s three types of TADS interpreters. Full multimedia terps (like htmlTADS and QTads), text-only UI terps (like Gargoyle), and text-mode terps (like t3run.exe and FrobTADS.)
You only need to worry about the second class: text-only UI terps. Gargoyle for example will only every use the single font configured by the user. And almost always, that’s a proportional font.
The other two terp classes will work fine. Multimedia terps will switch to a typewriter font if that’s what you specified in your HTML tags, and text-mode terps will always use fixed-width fonts for everything by definition, since they run inside a terminal.
However, you should be aware that
TADS-typewriter is not actually guaranteed to be a fixed-width font! I mistakenly assumed that this must be fixed-width in older versions of QTads, but in the latest version every font is allowed. That’s because many typewriter-style fonts you can find on the net are proportional, as this is needed in order for them to more accurately emulate the look of a real, physical typewriter font face.
If you want a guarantee that you’ll get a fixed-width font, wrap that text inside
</tt> tags instead. This will use the interpreter’s “Fixed-width Font” setting instead of the “Typewriter Font” one.