Any class power can be taken by a character if:
- That character can take powers of that class
- The class power in question has an explicit level and the character is eligible to take powers of that level. (Either from a newly gained power, or from retraining)
- The class power is of the same type (attack or utility)
This goes for Essentials characters taking regular powers and vice-versa.
The Slayer (I'm assuming this is what you meant by "Essentials Fighter" gains, at level 1 two stances (out of 6 to chose from). These are all listed as Fighter Utility
powers, so they are not eligible for a regular fighter to take. Compare to the powers gained at second level which are listed at Fighter Utility 2
. Those are eligible for any fighter to take.
The rules for this are on p57 of the Heroes of the Fallen Lands book.