You don't say when in Dec you are going, and it's almost the end of Nov now so there is not a lot of time. The quick and easy answer to your original question is yes, of course you can do the whole course while you are in PDC. I hate to discourage you from taking classes down there, but if you try to do it all in 6 days you will either be mentally and physically exhausted, or not get adaquate education and skills, or (more likely) both.
If you read the questions posted on the New Divers and Those Considering Diving Forum, you will see that not everyone finds the courses easy and learns every skill right off the hop. I struggled with some of the in-water skills when I did my OW, and that's apparently not uncommon. I took more than the minumum number of pool dives to practice the skills sufficiently to feel comfortable with them.
Then there is the theory. OW classes come with a textbook - the PADI book is about an inch thick. There is a test on each section and then a final exam. It's a hell of a lot to cram in just a few days, esp if you are trying to do the dives as well. You will spend a huge chunk of your vacation time in a classroom and/or in your hotel room reading and studying instead of enjoying yourself.
You could also take the referral route - taking the theory and pool sessions at home and just the test in Mexico. I got burned on that, too. I booked my OW dives with what I believed to be reputable dive shop but they cut corners and did not complete the required tests, so I ended up redoing my OW dives at home anyway when I got back just to feel honest and safe. Walter's suggestions re: screening the instructor/dive shop are valid.
My suggestions: 1) If you're not in a terrific rush, delay the whole thing and do it properly at home later on when you have more time and patience than on vacation. Maybe just do another DSD or 2 while you're in Mexico.
2)If you are really set on doing your OW in Mexico, go to the Mexico forum and get recommendations for a good instructor.
3)Then try to do at least
something before you go - get a copy of the text and start reading, do at least 1 or 2 pool sessions if you can fit them in, whatever you have time for. At least that will give you a head start.
Good luck! You won't be sorry once you are certified, but make sure you do it safely.