Do I have to use the matching shampoo?
No, though the scents are designed as pairs and can clash if mixed. The fragrance free versions sidestep the question entirely.
Where should I apply conditioner?
Lengths and ends rather than the scalp. The scalp produces its own oil; the ends are the part that needs help.
Is the 5L worth it for one person?
Per wash it is much cheaper, but it is a large container and takes a while to get through. Decant into a smaller bottle and keep the rest sealed in a cupboard.
Do you have anything for sensitive skin?
The fragrance free range is the place to start, as fragrance is the usual culprit. Hope's Relief is worth a look for particularly reactive skin.