My husband appointed me as legatee of the usufruct of his assets. He has had children from a previous marriage. Can I combine this legacy with my statutory right to 25% in full ownership?

No. Either the value of your legacy is lower than your statutory rights (Articles 757 and 757-1 of the Civil Code), in which case you may claim a supplement, or it exceeds them, in which case you may retain your legacy within the limits of the special disposable portion between spouses provided for under Article 1094-1 of the Civil Code (Civil Code, Art. 758-6).