Serums are applied first because they are lighter and absorb quickly. Moisturiser is applied after to seal in hydration and keep the skin feeling comfortable through the day or night.
Yes, many people with normal skin introduce anti ageing or collagen based products gradually, usually at night. Using them a few times a week allows the skin to adjust without changing the core daytime routine.
A moisturiser for normal skin is best used twice daily, morning and night. Morning use prepares the skin for sunscreen and makeup, while night application supports comfort after cleansing and daily exposure to pollution and indoor air conditioning.