A bit more stability than you need is better than a bit less. Apart from diameter, weight and length of the slug, the nose and base shapes need to be considered. So, the diameter, weight and length are not enough info to calculate bullet stability, strictly speaking. The relative position between the center of gravity and center of dynamic air pressure is what matters, and these are also affected by shape. And by, if there is a hollow point or not. And the diameter, depth and shape of such a cavity. Ditto, for hollow base projectiles.
If you were talking about waisted pellets, they are already aerodynamically stable and need very little spin. Spin for pellets is mainly to null out the effect of imperfections in shape and the placement of the CG relative to the primary axis. More spin helps pellet stability as it emerges from the muzzle, and is kicked by the air blast from behind. Air blast is something that can be mitigated for pellets via an effective air stripper. Overspinning pellets tends to make them spiral at longer ranges; which seems like a contradiction regarding stability.
Barrel twist requirements for a bullet peaks at a muzzle velocity right at the speed of sound. You are well above a stability factor of 1.5 at 900 FPS, so if your bullets shoot well out of your Texan, I would not worry about anything. Shooting PCPs at 1150+ FPS is silly for many reasons, that include excess air consumption and noise. So, simply don't go over 1050 FPS and you should be fine. If you want more power, shoot heavier slugs. Ones that still go fast enough to give you a flat enough trajectory for your range and mission.
