Going by the 25 count per inch, I assume your stitches will be about 1 mm each, which is extremely tiny for any kind of cross stitch embroidery. I doubt you can get a fabric which is fine enough to meet that thread count and still be loose enough to embroider over and take out the threads afterwards.
I would go instead to stabilizing the fabric itself on its back, with interfacing fabric or 'non-woven material' which you can leave on after embroidering.
Select a quality shirt which has the grain of the fabric straight and wash it so you are sure that it will stay that way before you spend the effort of working on it. Make sure you have the fabric completely flat and straight, and iron it on with the advised heat, so it sticks the best it can.
Work you embroidery with the material in a ring (or other frame) so it can not stretch at all while working. Stretching it will release the fabric attached to the back. Use the knit of the fabric as your counting guide.
After you have finished the embroidery you can take away all the interfacing that is not actually in the embroidered area, but be careful not to damage anything in the process.
I would advise to go for a different process myself. I would embroider on a suitable fabric and applique that onto the shirt. This way you can save the work once the shirt is worn out and transfer it to a different garment.