Abstract: Rare-earth samarium has been identified as a key stabilizer of the ferroelectric layered bismuth-oxide structure, but whether other lanthanides can play an equally decisive role remains an open question. This study shows that various lanthanides successfully stabilize the structure, demonstrating that the stabilizing role is not limited to samarium. The structurally optimized substitution level decreases with decreasing ionic radius: 25% for large La3+ versus only 6% for small Lu3+, achieving a 76% reduction in dopant concentration while maintaining phase-pure ferroelectric properties. This inverse relationship indicates that minimal doping suffices when ionic radius mismatch is minimized, providing a structural guideline for reducing dopant usage while preserving ferroelectric functionality. X-ray absorption spectroscopy of the Nd-substituted system provides spectroscopic evidence consistent with local lattice distortion and reveals coexistence of ferromagnetism and ferroelectricity in Bi1.65Nd0.35O3 films. These findings expand the functional capabilities of the layered bismuth oxide system and broaden its potential applications in future devices.