probably plural is Valdezes, although it's not exactly set in stone, seeing as it's a non-english word. What's happening is that you're applying a fairly general english rule to a close-enough case and english is flexible enough for that to be acceptable. However, you should note that it is technically correct to use an apostrophe for "hard to pluralize cases", notably P's and Q's, so if you had a violent objection to integrating a spanish word into a spanish-english hybrid plural, then an argument can be made that Valdez's is correct (or at least not incorrect, if you're taking Who Wants to be a Millionaire to court after getting the final question wrong).

style guides generally suggest writing the possessive s where it is pronounced, so possessive Valdez's rather than Valdez' for the singular, but really it's optional

but yes, the Valdezes' and the Goodriches' (house)