Can I speak English or Cantonese in Xiamen?

Yes, you can. Southern Fujian Dialect is mostly spoken in the city. People speaking Cantoese can understand their dislect more or less. Some young people may speak good English, but it is not widely used. Moreover, it is best to speak Chinese Mandarin if you can.

Actually most of Chinese in Xiamen doesn't speak English nor cantonese. There are some areas like macro polo areas and xiamen university, you may be lucky to meet some English speaking Chinese.