Can I use this shampoo every time I wash my hair, or should I alternate with a deeper cleanser?

The surfactant system is mild enough for frequent washing — even daily — because it respects the scalp's hydrolipidic film. You don't need to rotate it out for a "clarifying" shampoo unless your routine includes silicone-rich serums, oils or heat-protectants that don't rinse cleanly with mild surfactants. In that case, once every two to three weeks, swap in a deeper cleanser (one with stronger anionic surfactants and no conditioning film) to reset the hair surface. After that deeper cleanse, follow with a moisturising mask and return to BISOU Glossy for the next washes. If you only use lightweight leave-ins or no styling products at all, there's no need to alternate; the shampoo's conditioning agents don't build up in a way that dulls hair.