It is a closing window: a shorter scale length for a low pitch force the player to choose between reduced tension and increased inharmonicity, or to look for a delicate balance between the two.
Ideally, a string built to be inherently more flexible than usual might be the ticket, but the nonstandard length requirement of the Mini-P is an additional complication...
If I had one, I would either order a round-core Newtone set with a 115 or 120 low E or a Pyramid flatwound one with a 110, but I'd also consider modifying the darn thing so that it can accept long-scale strings. I'd be especially curious to try the .118" low B from the 5-string La Bella Low Tension Flexible Flat set (LTF-5A).
If one manages with a standard or thin gauge E that's fantastic. However, I wonder if any of you uses 5-string basses. I would guess most folks don't venture beyond a certain fret on the low B, for reasons of tubby tone (primarily due to the pickup position being closer to the centre of the vibrating string) and inharmonicity (chorusy tone), but if said fret is high enough (say the 9th? 10th?), it means that a usable region between the 3rd and said limit does exist, in which you do indeed already play a 28.6" scale with a 120 to 135 string (which would convert to: open to 6th or 7th fret on the Mini).
To the opposite end of the spectrum, I guess there are players who in fact do not use the B string past the 4th or 5th fret, or just occasionally, or who plain don't own a fiver precisely because they dislike the tone of the fifth string, among other reasons. To those, my argument is invalid. Plus, the same diameter on a 28.6" scale, compared to 3rd fret of a 34", means less total string stretching upon the attack, and a somewhat firmer feel, which probably contributes to standard/light gauges working at all on a short-shorty.
