Is deferred enrollment normal or acceptable to graduate programs? What are the acceptable reasons for it?
Many (but not all) graduate programs will let you defer enrollment for a semester or a year.  At the institutions I am familiar with, they did not particularly care about the reason for the deferral; in fact, some of them did not even ask, except in the most general terms, why the student wanted to delay entering the program.  People understand that students may have other obligations and things that they want to do, and taking on an internship would be a perfectly normal reason for a deferral.

To find you whether a particular program will let you defer acceptance for a year, you will have to contact the program.  However, assuming they allow deferrals, what you are proposing does not sound like it should be a problem.