I don't think it hurts anything to vaccinate again against the same virus. I had to do this in order to get into college; I'd had the same vaccine when I was a child but it was in a public-health setting, so apparently my health record doesn't have it. My doctor didn't indicate it would be a problem in any way to get vaccinated again, though he did think it wouldn't do me any further good.
I found some things that say you can mix them, but that you should wait 6 months. I haven't found anything explaining why.
This one says 3 months - https://www.zawya.com/mena/en/life/story/COVID19_You_can_now_take_Pfizer_vaccine_after_two_doses_of_Sinopharm_in_Abu_Dhabi-SNG_215089705/
I'd wait as long as you can before getting the Pfizer.
I don’t think there is an official answer to this. The vaccines are different from each other. You may have more reaction. It’s almost like being vaccinated against two different diseases.
This I don’t see any harm in taking the second shots after a short break.
I don't normally talk about tjhis, but I had Johnson and Johnson in the USA, because when I got there, that's what they were giving. The country I want to visit currently only accepts mRNA vaccines, so 6 weeks after the J&J, I got Pfizer. I again had a sore arm but this time forafter the first shot it lasted for 2 days and I also had a moderate headache for 2 days. I don't care, a teeny tiny price to pay. The second shot gave no side effects. I think they say things like 6 months wait because that's the sample they use for the study and they won't say more than what the study showed. I'm no immunologist but the reason I waited was to give time for the first to work. But I figure a month later it was done doing its thing.
Just got some new info last night. Fauci gave one of his many interviews, and he said although some docs are giving boosters of a different vaccine at this time there is no data on whether it's safe to do that and until there is, he won't recommend it. Of course he's a researcher and very data driven, whereas clinical docs often don't do a lot of homework and just go by their gut, which is often right and often wrong. Fauci always waits for data before he'll give an opinion, and that's what scientists do. Doctors who only do their daily practices aren't scientists. He's guessing it's probably safe to take two different vaccines, but again, there is as yet no studies or trials to go by. He was referring to the US, so he was talking about those who got the J&J and want the Pfizer or Moderna because of concern over effectiveness and the variants, but it applies to all the vaccines out there -- nobody has apparently done a study on what effects if any occur if you get two different ones. So the answer is, dunno.