There are indeed _strings_ of short scale, that can sound nice and full, _in the right circumstances_.
My E-cello has 28" scale and I have had a custom F and Bb string made for it. That is, F a half step above low E on a 4 string bass, and Bb a half step below the low B on a 5 string bass.
They are definitely more plunky than an electric bass at 34+ inch scale. They are definitely "floppy", very low tension. However they are entirely usable and sound a lot like an upright bass. You won't get E-bass 10-second sustain when plucking them, but there are plenty of creative ways to use these notes.
HOWEVER, you're not likely to get the same resonance/sustain on an acoustic cello, in this range, as you do on an upright bass. Even in normal tuning, pizz on a cello doesn't _generally_ sustain as well as on an upright bass. most violins, violas, and cellos give you a quickly decaying "plunk" when plucked. Some of it has to do with the size of the body. Some of it has to do with bridge design, and bracing of the insrument. Some of it is because for the upper members of the bowed string family, that's the sound _desired_ in classical music, so they're made that way on purpose.
I once had an acoustic cello that _did_ sustain very well on pizz notes, a William Harris Lee cello. It was a bit freakish compared to other cellos, in that regard; I've not since had an acoustic cello that does this well. It behaved more bass-like than other cellos--its low end was rich, the upper range did no project as well. I'm still sorry I sold it.
I see no reason a nylon or other synthetic string _couldn't_ be made that works. But you're really on your own making it work. You _need_ a certain amount of tension in your strings to hold the bridge in place on an acoustic instrument, or the sound post is in danger of collapsing when the instrument is bumped. I have doubts as to whether 4 rubber or nylon strings will be effective in that regard.
It really comes down to, how much fiddling are you willing to do? Why do you want to do this to a cello--because you want to put an otherwise wasted cello to use? Because you want to explore shorter scale for its own sake? Because you want to tinker with strings? I'm not mocking those reasons; those are all legitimate reasons to try this kind of experiment.
You _can_ get low F strings for 5-string cellos; that's a half step higher than the low E on an upright bass. Those are a semi-standard inventory item on the strings websites. Perhaps you might enjoy tuning it in 5ths, low to high F - C - G - D ? It would be a lower barrier to entry, and would involve things you can get off the shelf.
I'd say that you could even tune that F string down to E, and get the rest of the strings in "heavy" gauge (cello strings are available in light, medium, and heavy gauge), and tune it a half step down: E - B - F# - C#.
That seems an easier way to get a cello useful for playing bass lines.