how x forwarder will work for private to private ip

The X-Forwarded-For header is added by FGT whether the client IP is private or public.

Technically, the x forwarder (X-Forwarded-For header) is to record the original IP address of the client.  So it doesn't matter whether this is a private or public IP. And it should not impact your traffic flow as well since it just for a tracking purpose. 

Without it, you just do not see the client's original IP address.