I don't think they're doing any harm, and the slugs are probably helping to break down the compost. I'd leave them to get on with it.

That's an interesting combination! Ants in compost heaps usually means it too dry, because ants hate damp or wet conditions. If the slugs are on the top, then probably the heap itself is too dry, and the slugs have ventured in with some damp weather. I'd turn the heap and check it, and if its dry, give it some water. And not really worry about the slugs.

many thanks louisa15 and bamboo