Right additive identities that define addition completely:
x+0 = x
x+y’ = (x+y)’
where 1 = 0′, 2= 1′, etc.
This defines addition from counting by one. So addition is derived from counting by one. The term successor just means counting by one. Successor may be too fancy a term that hides the meaning of the definition of addition in terms of successor.
There is no valid reason not to teach the above two equations to students in K-6. In fact, it should be taught as soon as the plus sign is taught.
Letters can spell different words is taught. It is no more complicated to teach letters can stand for different numbers.
Six and seven stand for 6 and 7. If they can learn that, then they can learn that n can mean 6 or 7 and stand for either case.