what if the acronym was what susan made up, and the actual name, like car, boat, etc.. is really TARDIS, then Tardises would be the appropriate plural name, err.. I think?? right??
Well, if it really were a Gallifreyan word, then who knows how they form their plurals? But as an English word, Tardises or TARDISes would be right in any case -- and it is what they use on the show.
Speaking of back-forming acronyms, not long ago a pair of theoretical physicists came up with a relativistically valid mathematical model for a type of time warp which they called a Traversable Achronal Retrograde Domain In Spacetime. Here's their paper on the subject:
And here's the white paper explaining it for laypeople: