Tummy, I wouldn't begin to tell you to go against your dr's orders. You might be able to ask, if you are already passing clear liquid would that be enough. I personally have never had to drink the whole gallon of stuff before I was clean, but like I said before I actually start eating soft foods an extra day early. Your constipation may make it all necessary though. I actually don't remember the Mag/Citrate routine, so I can't comment on that. I just know I used it once. I will confess that I start drinking the gallon stuff a little earlier (couple of hours) than told and give myself longer to drink it. It's not that it taste that bad, it's just I wouldn't even want to drink a gallon of coffee or any other of my favorite beverages.
I know today is day one for your prep, so I hope it is going well. Pamper yourself as best you can; you deserve it!