Often I cook frozen oven chips and frozen fish (in batter) in the oven at the same time. The cooking instructions for the chips usually say 20 minutes (or less); the fish usually more than this. They require the same temperature, usually 220C Why is it that (if I follow the instructions) the chips are always undercooked and the fish is always overcooked? It's the same with any manufacturer's product; the coking times always seem to be wrong. I have tried using different positions in the oven but always the same result.
My personal experience with frozen oven fries is like Wayfaring Stranger's, it takes longer to cook them than what the manufacturer suggests. I think though this comes down to personal preference. If I do follow the instructions then I get fries that strictly speaking can considered cooked, they're hot throughout, but they can be fairly soggy and not anywhere near as crisp and brown as I like. So I generally bake them as long as they can stand, to the point where a few the tips are black and maybe one or two smaller ones have gone completely black and inedible. This takes the fries to the other extreme, arguably I've overcooked them and they're dried out, but that's the way I've come to like them.
In addition to simply cooking your fries until you think they're done, regardless of what the manufacturer says, you should make sure that you're baking them the correctly. The answers to the question How to bake Frozen French Fries have some good advice. In particular, it's important that the fries be spread out evenly in a single layer on a pan. Try not to crowd the pan too much, that will make them take longer to cook. Also flipping them half way through will help them cook more evenly and faster.
I don't have much experience with cooking frozen fish, so I don't know why they would end up being over cooked when you follow the manufacturer's instructions. You might try cooking them separately. You can bake fries in a toaster oven if you don't need to cook that many, and you can probably do frozen fish in one as well.