The only problem with feeding a variety menu to your corns is that you WILL eventually come across a corn that, once given the opportunity to taste something "exotic", will decide that it will accept nothing less. Furthermore, many cornsnakes die every year, not because they are unhealthy, but rather simply because they refuse to eat something that is cheap enough to keep them on.
I can't tell you how many baby corns are hatched out each year as confirmed lizard feeders, and many of them simply will not switch without INTENSE trickery/training to get them over to mice. Some will simply starve themselves to death in a room full of mice, waiting for you to give them an anole!
If you have a corn that is eating mice well, and you give it a quail, it may not do anything other than give the snake horribly foul (no pun intended!) smelling feces. BUT ... it may give the snake something that it simply will no longer do without, and the animal may go on hunger strikes without quail as its meal. That will get expensive!
My advice is to offer such "treats" sparingly, if at all. If you have a mice-feeding cornsnake that always takes that f/t rodent, count your blessings! Others are not nearly so fortunate, I assure you!!!
