I'd say, with the right "rush" attitude, a 50min int-int connection will work out (even if it's illegal) if the following happens:
-On-time arrival, ie doors open no later than 40min before departure (leaving you about 30min till boarding is closed) - quite good chance for this if arriving on a long-haul flight. Or during non-rush hours/months.
-Boarding passes for connecting flight in hand
-Knowing where to get upon arrival (left side of immigration, the transfer immigration document check counter, followed by security downstairs)
-Immediately making yourself known to the staff that you've a tight connection. At security, they actually got a monitor with a stuff next to it showing "tight connections" that are allowed to jump in front of the queue
-Obviously walking fast to and from transfer security