starsuperion wrote:
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 backforming 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:
http://arxiv.org/abs/1310.7985
And here's the white paper explaining it for laypeople:
http://arxiv.org/abs/1310.7983