Is sulfasalazine effective in treating hives/allergies?

Yes, but... Urticaria (hives) are often due to a food or medication reaction when acute & are usually easily managed with antihistamines. When hives do not respond to antihistamines or become chronic, anti-inflammatory medicines are often used. First-line treatment is usually with corticosteroids, but when hives persist, other anti-inflammatory agents including sulfasalazine have been used with some success.
NSAID. Usually not. It is a potent non-steroidal anti-inflammatory medication. In fact, it can actually make the hives worse.