You could try with:
AND (pc.products_id, pc.categories_id)
IN
(SELECT
pc.products_id, MIN(categories_id)
FROM
zen_products_to_categories pc
GROUP BY
pc.products_id)
Edit:
In MySQL a subquery like this is usually slow. You should have better luck with a JOIN:
SELECT ....
FROM
....
INNER JOIN (SELECT
products_id, MIN(categories_id) min_categories_id
FROM
zen_products_to_categories
GROUP BY
products_id) min_ct
ON pc.products_id=min_ct.products_id
AND pc.categories_id=min_ct.min_categories_id
WHERE
....