IMO the lowest you can tune a regular 25.5 guitar is Bb, maybe A.
You have to put on quite thick strings to compensate the short string length and it will start to stiffen up. They do not resonate good enough and the harmonics will go off... It won't intonate properly.
I tuned a 27 scale guitar to G with a 0.74 string and it was okayish... Ab would have been better with a lighter string, maybe 72 or 68.