as the fixed pitch were supposed to be more stable
Whatever gave you that idea? FP are less stable, due to their lower head speed. FP helis do have advantages (robustness, simplicity, less likely to damage furniture, less costly to repair) but I would not count stability among them.
I was thinking of reducing the pitch on the blades to allow the motor to spool up to a higher rpm
This is a common way to add stability (and also improves behavior outdoors) Cutting the trailing edge of FP blades is a very effective way to reduce pitch.