Is it okay for the groom to wear tails if the wedding is not in the evening?
Not really. Tails are more of a super-ultraformal evening wedding thing. If you like the formal look of a long jacket, however, think about a cutaway or morning coat for the groom. This is a very Victorian-looking, gray, often pinstriped coat, worn with a vest and ascot. It's very dapper, very Hugh Grant in Four Weddings and a Funeral. You should be able to check out the styles available at your local wedding formalwear store.
See More: Most Asked Wedding Questions , Groom Style , Groom Basics , Tuxedos + Attire , Attire for the Groom