How heavy is each flake? In my experience, alfalfa flakes tend to be on the smaller side.
I go by this rule of thumb if I mix hays:
Usual Hay- 1 flake
New Hay- 1/4 flake
If you give your horse 2 flakes of the other hay, then 1/2 a flake of alfalfa should be fine to introduce your horse with it.
Hay doesn't seem to be such a big shock to their system as grain can be. It's like a horse getting into a richer section of grass. Sure, they make experience runnier (hope nobody is eating dinner while reading this) poop, but it won't cause any serious problems.
If your horse is prone to colic or other digestive issues, then play it a bit safer if it makes you more comfortable.
Just don't worry so much about it. Horses can adjust.
For about a whole week, a barn owner where I boarded skipped feedings and/or double fed.
But the horses were fine. I wouldn't recommend that, though!