Rainbow Leucothoe is a great choice! It thrives in partial to full shade and can tolerate some morning sun, but too much direct afternoon sunlight might scorch the leaves. For soil, aim for well-draining, acidic conditions. It's important to keep the soil consistently moist, especially during dry spells, so mulching can help retain moisture. Pairing it with ferns or hostas sounds lovely, as they all enjoy similar conditions. As for pests and diseases, watch out for leaf spot and root rot; good air circulation and proper watering can help prevent these. Have you considered adding any ground cover to enhance the ensemble?