Niacinamide pairs safely with vitamin C, retinoids, AHAs, BHAs, peptides — almost every other common active. The old warning about it 'cancelling' vitamin C was based on lab conditions that don't apply to modern formulas.
The pairings that genuinely work
Vitamin C in morning + niacinamide in same routine = stronger antioxidant defence plus barrier support. Niacinamide + retinoid in evening = retinoid efficacy with reduced irritation. Niacinamide + acids on alternate nights = exfoliation plus barrier maintenance.
The few real interactions to know
Don't layer niacinamide directly with strong AHAs on the same step (vehicle pH conflict). Use them on alternate evenings instead. That's the only meaningful caution; the rest of the cocktail is fine.
Niacinamide at 4-5% in most modern serums is one of the most cost-effective additions to a routine. The layering flexibility makes it useful in almost any combination.