I only have had problems one time, that was flying out or Monterrey, Mexico on VivaAerobus. A lady checking people boarding the flight looked through my diabetic case with all of my insulin and syringes and told me I was only allowed 4 syringes and I had probably 25. I immediately forgot every word of Spanish that I knew when I started trying to tell her I was diabetic and took 4 or 5 shots a day and I needed that supply. A kind bi-lingual older gentleman behind me started helping me argue my case and she finally relented.
All to the other times I have flown I have not had any problems with being diabetic. I flew out of TJ for my honeymoon a year and a half ago to Ceurnavaca, Mexico and had not problems.
I do not use a pump but I don't think it should cause you problems.
I agree with the doctor's letter... it is a good thing to have with you, just in case.
Blessings from Ensenada, Mexico