What is the optimal setting for innodb_buffer_pool_instances for a mid level AWS RDS server with 6gb buffer pool?
Short Answer:  Not a problem.

Long Answer:

The "instances" feature is new (as of 5.5.4) and exists to speed up certain things when the system is running at peak load.  But even then, it does not help the performance more than a few percent.  For lightly loaded servers, the improvement is probably not measurable.

What about "8"?  I have yet to hear of anyone getting in trouble when each instance is less than 1GB.  I suspect it is "advice", not "a hard and fast rule" to make each instance at least 1GB.  In fact, glancing at a few dozen servers, I see that almost half of them violate that.  I see one machine with 10 instances for only 2GB of buffer_pool.

I would trust RDS to provide reasonable settings.  If their settings are bad, they will be bad for all their clients.  Imagine how much hassling RDS would get!